Reimplement SNAP standard deduction as 8.31% of the net income limit up to 6 people

The law https://www.law.cornell.edu/uscode/text/7/2014#e_1_A_i defines the standard deduction more simply than we've implemented it:

>(I) equal to 8.31 percent of the income standard of eligibility established under subsection (c)(1); but
(II)not more than 8.31 percent of the income standard of eligibility established under subsection (c)(1) for a household of 6 members.

[(c)(1)](https://www.law.cornell.edu/uscode/text/7/2014#c_1) defines the net income limit at 100% of FPL.

At first glance though, these numbers don't align. For example, the [FY2022 standard deduction per USDA](https://fns-prod.azureedge.net/sites/default/files/media/file/FY22-Maximum-Allotments-Deductions.pdf#page=2) for a 1-person household in 48 states and DC is $177/mo. But [HHS sets the poverty line](https://aspe.hhs.gov/topics/poverty-economic-mobility/poverty-guidelines) for a one-person household at $13,590. 8.31% of $13,590 is $1,129, or $94/mo. $177/mo is 15.6% of $13,590. @nmrodelo have you looked into this?

Once we figure it out, we can replace the current parameter, which is indexed by family size and state group, with two parameters: 8.31% and 6.