I have an estimated $100k in equity that I could pull out with a cash out refi if I wanted to refinance again and increase my savings to around $150k. 

The quick and dirty formula for a BRRRR isPurchase price = 0.75 * ARV - Rehab CostsIn your case, you would need to acquire the property for ~$156.25k to make it a complete BRRRR deal.Side note, rehab costs are very hard to estimate so make sure you verify that estimate and/or allow for a significant margin of error in your calculations.

For example, if lite rehab is needed estimate X per square foot (i.e. maybe paint, carpet, small touch ups and improvements).  

If you are just starting out, then also know that your estimates are going to almost always come in too low.

Ask your mother and Father what they want done about the house--he has been living there free for a reason--maybe they will just deed it over to him.Maybe they like him more than you--or maybe he's the kind of person that needs more help than you (in your parents estimation).He has been living there free and clear for a long time.Here's an odd thought--maybe you were adopted and don't know it (seriously).Somehow he received a free house and you did not.Also you might want to look into living trusts (done by an attorney). 

I’m going to offer 90k using a conventional loan 20% down and cover the rehab myself(estimated 40k). 

You must know the rehab budget and nail the ARV (it can not be a guess or estimate, it must be based on actual sold comps in your area.

Go watch some sales, see who gets the property, follow the eviction of whoever is squatting there, see how they secure the property, and fix it up, estimate on your own, talk to them, maybe get a job working for them

I estimate 350-500K in repairs and the ARV based on conservatives numbers at 8% cap is around 1.6 million.I have been "winging" it on my previous buildings to an extent but all of them turned out great with excellent cash flow and big forced appreciation. 

Back of the napkin, you can estimate them like so:Purchase price (or assessed value, whichever is newer) x .06 for a rental x millage which is often .45 or so, depending on the county and district.