If everything is brand new in the building, what am I really looking at for an estimated Cap Ex percentage when crunching my numbers?

Looking at 71,000 combined by my estimates of 4% on loan origination, and 8% selling.

After accounting for property management ($50/mo is what i pay) and reserves ($100/mo) i estimate this to cashflow about $350/mo.

I also over estimated costs, since this will be my first BRRRR and I have never flipped a home. 

I purchased a 2 level home 4 bed 3 bath so it has 3 bed 2 bath upstairs an 1 bed 1 bath downstairs it also has a like a rec room downstairs I am thinking about putting a kitchen in the rec room an renting out the downstairs my mortgage is 2900 I paid 500,000 for the home its estimated to be worth 710,000 on redfin I'm trying to cover some of my mortgage by renting out downstairs my question is if I had that kitchen downstairs can I do a cashout refi to get my money out of the deal its going to cost me about 13000 to do it plus make a door downstairs because there isnt a entry way downstairs also I wanted to put a washer and dryer upstairs because they have one downstairs so it would be like a 1 bedroom apartment all in at about 17000 can I consider this the brrrr strategy an refi after it's done an get my money back out to reinvest in another deal?

If you are going to buy a property as a BRRRR you will generally be able to have a much more detailed, much more accurate estimate of expenses, especially in the short term (5-10 years). 

What's a good way to test the accuracy of tools like the BP rent estimator and others?

Estimated that from SmartAssest property tax estimator. 

It was in poor condition with some of the following problems:No hvacPast termite damageWater damage in multiple areas with active leaksAluminum wiringPoor layoutGrading issuesFoundation issuesWe did some numbers and estimated the rehab to be close to $100,000.

But from my initial DD, their estimates of repair costs and comps used seem to have wild ranges.