Anything with a modified AGI will have a worksheet in the instructions detailing the calculations.

By ACA I assume you mean PTC.  That one's on page 8 of the instructions:

https://www.irs.gov/pub/irs-pdf/i8962.pdf#page=8

But NIIT was also born out of the Affordable Care Act.  It has a different MAGI calculation (just to use as an example that they are not all the same):

https://www.irs.gov/pub/irs-pdf/i8960.pdf#page=20

In general, the various MAGI calculations help to apply ordering rules to various tax breaks granted by Congress so as to avoid circular calculations.  Ironically PTC/SEHI is the exception to the rule and involves a circular calculation.