My head is not in taxes yet, but looking back at this thread, my advice (as long as Turbotax hasn't changed) is:
Use the Ferguson calculator referenced on this webpage, which is all about the circularity of the calculation. You will enter your income BEFORE any ACA deduction.
https://thefinancebuff.com/when-tax-software-gives-self-employed-wrong-aca-premium-subsidy.html
This is the procedure that worked for us last year. We told Turbo Tax that we DID NOT purchase our insurance through the marketplace (we did purchase it through the marketplace--but saying we didn't allowed us to get around TT glitches). I used Ferguson calculator to determine PTC and self-employment deduction based on our income. I used this info to enter self-employment deduction in business expenses, where by the way the guidance is NOT to do this (ignore). TurboTax then correctly figured premium tax credit on the PTC form. I made sure deduction and credit did not equal more than total premiums for health insurance. Make small adjustments to the self-employment deduction if they do.
