I suggested we re-pour the driveway, and factor the cost into the total expenses for the flip, my wife disagrees. 

We are interested in learning how to factor annual Tax Rate and End Value into our equation.Is there another way?

The other big factor he alludes to is also what @Richard Sherman pointed out. 

Matt,Speaking toward the insurance piece, your insurance premiums are going to depend on several different factors such fire protection class code, roof age, square footage, age of dwelling, etc.

Accounting may play a factor. 

You won’t get the volume pricing on materials new home builders get so factor in Home Depot retail pricing on everything too.A half burned down house will cost more to repair than building a brand new house from scratch.

Of course, there are often other factors about a company's reputation or how their estimator bid the job that causes me to break that guideline.I know with a lot of my subs, after I've given them the job, are often quite curious as to how the other bids came in.  

Other factors include large corporations and non-profits dumping money into the local community.

As with anything, you'll find people successful with it and some that are not successful - the factors above are what is important.Since you are using an IRA, make sure you have enough reserve funds in there to handle up front expenses or unexpected repairs.

I know of a community bank up here that has done 80% LTV on refi’s for multi-unit properties but again the rate and terms they offer will be vary based on several factors.