Or do you have formula or process for coming up with this.  

The property is going for a little more than what meets my formula and conditions and has multiple offers.  

I have a simple formula I apply to calculate the percentage a rent increase should be on a unit.First, look on craigslist/zillow for other advertised rentals similar to and nearby yours.  

Does anyone have links to (free) great ROI formulas, excel spreadsheets, or calculation sites?

A property that needs an extensive rehab that cost upwards of 70K needs much more thought out calculations and your 75% formula will not suffice.

So the question is, how do I account for the cost of rehab in the PB formulas?  

The terms @ 6.75% and 30yr fixed are very good.I think the more pressing issue is the inadequate funds needed for proper repairs.I am a believer in creating a formula that works for you.

I've posted a formula that I follow on selling my position or the deal all together in another topic. again, Nice deal!

I agreed with your formula except for that one part.

I like the formula that J Scott and others use where you back into what you'll pay:Expected ARV- Rehab Costs- Fixed Costs- Desired Profit-------------------------= What you are willing to payIf the ROI is not high enough, and you find problems you didn't anticipate, that additional cost can only come from one place - your profit.