12 C.F.R. § 324.143
(c) The SFA risk-based capital requirement.
(2) If KIRB is less than or equal to L, an exposure's SFA risk-based capital requirement is UE multiplied by TP multiplied by the greater of:
(3) If KIRB is greater than L and less than L + T, the FDIC-supervised institution must apply a 1,250 percent risk weight to an amount equal to UE · TP · (KIRB − L), and the exposure's SFA risk-based capital requirement is UE multiplied by TP multiplied by the greater of:
(d) The supervisory formula:

(e) SFA parameters. For purposes of the calculations in paragraphs (c) and (d) of this section:
(3) Capital requirement on underlying exposures (KIRB).
(i) KIRB is the ratio of:
(4) Credit enhancement level (L).
(i) L is the ratio of:
(5) Thickness of tranche (T). T is the ratio of:
(6) Effective number of exposures (N).
(i) Unless the FDIC-supervised institution elects to use the formula provided in paragraph (f) of this section,

where EADi represents the EAD associated with the i th instrument in the underlying exposures.
(7) Exposure-weighted average loss given default (EWALGD). EWALGD is calculated as:

where LGDi represents the average LGD associated with all exposures to the i th obligor. In the case of a resecuritization, an LGD of 100 percent must be assumed for the underlying exposures that are themselves securitization exposures.
(f) Simplified method for computing N and EWALGD.
(1) If all underlying exposures of a securitization are retail exposures, an FDIC-supervised institution may apply the SFA using the following simplifications:
(3) If C1 is no more than 0.03, an FDIC-supervised institution may set EWALGD = 0.50 if none of the underlying exposures is a securitization exposure, or may set EWALGD = 1 if one or more of the underlying exposures is a securitization exposure, and may set N equal to the following amount:

where: (i) Cm is the ratio of the sum of the amounts of the `m' largest underlying exposures to UE; and (ii) The level of m is to be selected by the FDIC-supervised institution.