This gives me a good base.Do you usually add some cushion to your numbers?

If I had more of a cushion, then I might try making that offer.

It could be As simple as a specialty adhesive cushion/caulking type product applied before putting new drywall up, or maybe rubberized mounting strips or something.

Considering I do not have a lot of cushion should something go wrong early on.

How long does it take before that helps you pay down some of your debt or create a financial cushion?  

Good DSR can cushion a job/income change, new purchases and on and on.

Based on what we find (Let’s say $40,000 Rehab estimate- including a 20% cushion) I’ll determine my offer this way:  $250,000 ARV x 70% = $175,000 My All-in Costs basis

I certainly understand your mindset, however I would go with the 2nd rental property rather than paying off the mortgage right away.You might initially have less cash flow, but rental income should increase while your mortgage payment should only be decreasing (assuming you don't have an adjustable rate mortgage).I also like the idea of having more than 1 rental property because, if each property is cash flowing like they should be, you have more of a financial cushion during periods in which one property is vacant.

The percentages get really skewed too the more your ARV changes.  15% profit on a $100K sale ($15K) does not leave much room at all for error and is too thin, IMO.  15% profit on a $700K sale ($105K) has more cushion. 

Those reserves served us well in 2008-2010 as both a cushion for negative cash flow events and sometimes as opportunity fund.We're at about 60% leverage overall.