You should receive a "EligibilityResultsNotice.pdf" when you sign up for an ACA plan. You can find this in your Messages in your healthcare.gov account. This should list both your max credit and the MAGI used to calculate the max credit. Use the MAGI in IRS form f8962 to calculate your monthly contribution. Add your monthly contribution to the max credit from the notice. This should be the approximate SLCSP value.
Caveats:
1. Form f8962 uses a table to calculate your monthly contribution factor. This table may change slightly from year to year or change largely if tax laws change. Small changes are generally insignificant. The 2021 tax law change was significant and resulted in a new EligibilityResultsNotice and change to SLCSP. If doing this in December of year X, form f8962 for year X should be available by then.
2. The EligibilityResultsNotice is a healthcare.gov thing. Not sure what you get from state ACA exchanges.
