After reading all of your replies (and thanks for all the good discussion), I see I can back into the SLCSP used to determine my APTC subsidy.
I went back to the confirmation letter I received from my state's (NY) exchange to remind me of the estimated MAGI I reported to them a year ago (I needed to show proof again due to a technical reason, but that did not affect my ATPC subsidy, nor was it different for January versus subsequent months).
I plugged the estimated income I reported into a spreadsheet which mimics the calculations shown on Form 8962, along with the appropriate FPL. When I included the SLCSP amount I received a few days ago, the actual and calculated subsidies were nearly identical, about $2 apart, as SecondCor521 suggested.
This means, as Cathy63 suggested, I can do for next time, by trial and error, enter the MAGI and FPL amounts (are the FPLs published somewhere in advance?) into my spreadsheet, then the APTC amount. I can then enter different SLCSP amounts to see which one gets me to a zero difference between the actual APTC and the calculated one. This does assume that the %-of-MAGI figure from that big table (Table 2) changes little or not al all from the prior year. Some years, it changed a little. It remained unchanged from 2021 to 2022.
This will, at least, greatly reduce any unpleasant surprise I see with the SLCSP, as it is known by my state's exchange at the start of the year even though their phone reps have no idea what it is.
Thanks to you all for an educational discussion which will help me going forward. I can build this into my 2023 tax spreadsheet.