Purchase Price for Flips

[Cyrus Sidhwa]

The standard formula for max purchase price is 70% of ARV minus Repairs. However, this doesn't seem to account for closing costs or holding costs which can easily reach tens of thousands of dollars when using a HML. My initial thought is to make the formula: 70% of ARV minus (Repairs + Holding Costs + Closing Costs When Buying + Closing Costs When Selling). However, I imagine deals that meet that criteria are extremely rare.

I am wondering if the following makes sense, or if there are flaws and/or things I'm missing / not accounting for. Here's my formula: ARV minus (Minimum Acceptable Profit + Repairs + Holding Costs + Closing Costs When Buying + Closing Costs When Selling). Does anyone use anything like this?

I would appreciate any experienced flippers explaining how you build the additional costs into your max purchase price formula. Thank you!