5 U.S.C. § 8452 : US Code - Section 8452: Computation of disability annuity

      (a)(1)(A) Except as provided in paragraph (2), or subsection (b),
    (c), or (d), the annuity of an annuitant under this subchapter - 
        (i) for the period beginning on the date on which such annuity
      commences, or is restored (as described in section 8455(b)(2) or
      (3)), and ending at the end of the twelfth month beginning on or
      after such date, shall be equal to 60 percent of the annuitant's
      average pay; and
        (ii) after the end of the period referred to in clause (i),
      shall be equal to 40 percent of the annuitant's average pay.

      (B) An annuity computed under this paragraph - 
        (i) shall not, during any period referred to in subparagraph
      (A)(i), be adjusted under section 8462; but
        (ii) shall, after the end of any period referred to in
      subparagraph (A)(i), be adjusted to reflect all adjustments made
      under section 8462(b) after the end of the period referred to in
      subparagraph (A)(i), whether the amount actually payable to the
      annuitant under this section in any month is determined under
      this subsection or otherwise.

      (2)(A) For any month in which an annuitant is entitled both to an
    annuity under this subchapter as computed under paragraph (1) and
    to a disability insurance benefit under section 223 of the Social
    Security Act, the annuitant's annuity for such month (as so
    computed) shall - 
        (i) if such month occurs during a period referred to in
      paragraph (1)(A)(i), be reduced by 100 percent of the annuitant's
      assumed disability insurance benefit for such month; or
        (ii) if such month occurs other than during a period referred
      to in paragraph (1)(A)(i), be reduced by 60 percent of the
      annuitant's assumed disability insurance benefit for such month;

    except that an annuity may not be reduced below zero by reason of
    this paragraph.
      (B)(i) For purposes of this paragraph, the assumed disability
    insurance benefit of an annuitant for any month shall be equal to -
    
        (I) the amount of the disability insurance benefit to which the
      annuitant is entitled under section 223 of the Social Security
      Act for the month in which the annuity under this subchapter
      commences, or is restored, or, if no entitlement to such
      disability insurance benefits exists for such month, the first
      month thereafter for which the annuitant is entitled both to an
      annuity under this subchapter and disability insurance benefits
      under section 223 of the Social Security Act, adjusted by
        (II) all adjustments made under section 8462(b) after the end
      of the period referred to in paragraph (1)(A)(i) (or, if later,
      after the end of the month preceding the first month for which
      the annuitant is entitled both to an annuity under this
      subchapter and disability insurance benefits under section 223 of
      the Social Security Act) and before the start of the month
      involved (without regard to whether the annuitant's annuity was
      affected by any of those adjustments).

      (ii) For purposes of applying section 224 of the Social Security
    Act to the assumed disability insurance benefit used to compute the
    reduction under this paragraph, the amount of the annuity under
    this subchapter which is considered shall be the amount of the
    annuity as determined before the application of this paragraph.
      (3) Section 8462 shall apply with respect to amounts under this
    subsection only as provided in paragraphs (1) and (2).
      (b)(1) Except as provided in subsection (d), if an annuitant is
    entitled to an annuity under this subchapter as of the day before
    the date of the sixty-second anniversary of the annuitant's birth
    (hereinafter in this section referred to as the annuitant's
    "redetermination date"), such annuity shall be redetermined by the
    Office in accordance with paragraph (2). Effective as of the
    annuitant's redetermination date, the annuity (as so redetermined)
    shall be in lieu of any annuity to which such annuitant would
    otherwise be entitled under this subchapter.
      (2)(A) An annuity redetermined under this subsection shall be
    equal to the amount of the annuity to which the annuitant would be
    entitled under section 8415, taking into account the provisions of
    subparagraph (B).
      (B) In performing a computation under this paragraph - 
        (i) creditable service of an annuitant shall be increased by
      including any period (or periods) before the annuitant's
      redetermination date during which the annuitant was entitled to
      an annuity under this subchapter; and
        (ii) the average pay which would otherwise be used shall be
      adjusted to reflect all adjustments made under section 8462(b)
      with respect to any period (or periods) referred to in clause (i)
      (without regard to whether the annuitant's annuity was affected
      by any of those adjustments).

      (c) Except as provided in subsection (d), the annuity of an
    annuitant under this subchapter shall be computed under section
    8415 if - 
        (1) such annuity commences, or is restored, beginning on or
      after the redetermination date of the annuitant; or
        (2) as of the day on which such annuity commences, or is
      restored, the annuitant satisfies the age and service
      requirements for entitlement to an annuity under section 8412
      (other than subsection (g) of such section).

      (d)(1) The annuity to which an annuitant is entitled under this
    section (after the reduction under subsection (a)(2), if
    applicable, has been made) shall not be less than the amount of an
    annuity computed under section 8415 (excluding subsection (g) of
    such section).
      (2) In applying this subsection with respect to any annuitant,
    the amount of an annuity so computed under section 8415 shall be
    adjusted under section 8462 (including subsection (c) thereof) - 
        (A) to the same extent, and otherwise in the same manner, as if
      it were an annuity - 
          (i) subject to adjustment under such section; and
          (ii) with a commencement date coinciding with the date the
        annuitant's annuity commenced or was restored under this
        subchapter, as the case may be; and

        (B) whether the amount actually payable to the annuitant under
      this section in any month is determined under this subsection or
      otherwise.