@Kevin Keene ,
Here are my thoughts:
First off, you should definitely NOT be relying at all on Zestimate for values.
Secondly, it took me a bit to figure out what you were doing with the ARV calculator formula. Yes, you could rearrange the formula they way you did and it makes mathematical sense, but I think it messes with the psychology. If you say, "I can get the house for $150K so the ARV better be $278.5K to make it work" I worry you would end up fudging numbers a bit to make it work because it seems a bit more committed already.
Instead, you should approach it the way the formula is stated. Find ARV (again, not from Zestimates), estimate repairs and then determine max allowable offer. The property has to be purchasable at that price, or you don't do that deal.
I know it seems like semantics because, yes, you can rearrange the formula but if you have the foregone conclusion of what the purchase price is, you could be tempted to fudge repairs or choose slightly better comps to make it work, but if the comps and repairs are figured out objectively, then you can't argue with the MAO.
Hope you follow my thinking.