pro rating the SSTWB integration limit in a short year (plan term)

I'm doing a final PS allocation for a calendar year plan that was terminated 9/15/23.  That was the final pay date before they were purchased, and all payroll stopped then, and the purchase agreement required that the plan be terminated as of that date.  I couldn't convince them to let the plan run until 12/31/23.

The PS formula is integrated at 80% SSTWB + $1.  So for 2023, if it was a full calendar year, that would be $128,161.  x8.5 /12 = $90,780.71 (or something like that, depending on when you add the extra dollar).  The question is what to do with the integration level.  This is below 80% of the annual limit, so do I have to use 4.3?  Or do I get to still use 5.4% because it's 80%+ of the applicable limit once the pro rating is considered?  My software gives me an error on the latter because 56.667% of the TWB is not at least 80%, but I know to always double-check things in tricky situations...

See 1.401(l)-2(d)(5) (the 401(l) is an "L"). https://www.law.cornell.edu/cfr/text/26/1.401(l)-2

It looks like you calculate the amount of the integration level using the 80% SSTWB + $1 (see 1.401(l)-2(d)(3) which has a maximum excess allowance of 54%.

The integration level is pro-rated over the number of months (which I understand includes the partial month) in the short plan year.  This is your calculation to get to  $90,780.71. 

The last sentence in 1.401(l)-2(d)(5) says "No adjustment to the maximum excess allowance is required as a result of the application of this paragraph (d)(5), other than any adjustment already required under paragraph (d)(4) of this section."

I read that sentence as saying that you use the integration rate that fits with whatever you pro rated to.  So in my case, 57% is in the 4.3% range, so I use 4.3%.  If you were pro rating in, say, November, you'd probably still be above the threshold to get 5.4%.

Thanks for the cite.