I had trouble finding a vendor due to a harsh winter last year. so I approached a vendor that had given me an estimate back in April 2021 (when I purchased the property). 

So to come up with my offer on a house to present to a cash buyer, I use the following equation:ARV x Investment Percentage - (Renovation Estimate) = Cash Buyer OfferSo I'm trying to figure out how people come up with the investment % the use in the equation.

Advertised Cap rate is 12.5%, although that will change as I'll have to add management costs into the deal (estimating 8% of total annual rent for this, please let me know if it should be higher than this). 

ServPro did complete their work by the first week in October, and I was able to have my GC provide me with an estimate for all of the put-back.

They are all projections of estimated returns. 

The property has space for approximately 27 units.And I have no idea how to estimate monthly operating costs.

Then let the potential buyer know the situation and that the estimated buy-out is already included in the numbers you've presented. 

I’d like to build a model that would be easy to plug in a property’s estimated numbers and it tell me if it will be a better flip or buy and hold investment.

@Jill Jay I believe the best person to contact to begin would be a General Contractor for an estimate if you aren't doing your own work.

First of all congrats on your first purchase   Call a couple contractors and obtain their thoughts and estimates to compare repair costs.