when you state 'tax exempt SS', what are you referring to? are you referencing the 15% that is not taxable (up to 85% is taxable depending on total income).
look at the IRS instructions for form 8962 at their website and on the top of page 6 (on the right) is a table to calculate MAGI. if you look at line 4 "Form 1040 filers' and work through that instruction, what it requires you to do is remove SS taxable income and replace it with gross SS income.
Hopefully, that should be the 'official' documentation you are seeking!