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. 

My favorites are The Book on Managing Rental Properties by Brandon Turner and the The Book on Estimating Rehab Costs by J Scott.House-hacking is one of the best ways to get your first property AND gain valuable experience.