It's not really a conversion. It's a formula. GPA + Class Rank + Plus course difficulty/availability +..... + ACT = Academic score. But in-BETWEEN each of those "+" signs are formulas for calculating a composite score. If the SAT is used instead of the ACT, then the formula changes at that point.
You can't do a direct conversion with your "28", because there aren't the same number of tests. ACT has 4 or 5 (depending if you take writing). SAT has 3.
There is a conversion that some schools use; e.g 28 composite ACT is equal to 1880 CR+M+W; or a range of between 1860-1910. Unfortunately, it's difficult to say it's a direct correlation. But when used in the formula, it equals. I.e. a 36 on each ACT test and composite, once formulated, would equal an 800 on each SAT score. But you can't have a direct number comparison the way you're asking. It depends on each of the individual tests and the formula. Not just the composite.