These variations in assumptions surprisingly have a meaningful impact on your returns / interest payments over the course of the loan.

Finally,  there is no exact science ( as evidenced by the fact that you will probably have numerous opinion variations to this question and if you have 500 CPAs in a room and ask them, you will probably have 500 opinion) so the easiest way, if you are unsure and you prepare your own tax returns, I would keep it simple.  

The formula we use to calculate ROI is as follows:ROI = ((Income - DebtService - ManagementFee - Insurance - RETax - PeriodicFees) x (1 - StateTax)) / ( DownPayment + ClosingCosts + RehabCost)While there could be dramatic differences in factors like maintenance, vacancy rates and such, I will keep it simple and only show the impact of variations in state income tax, property tax and insurance.Below are details on the example property I will use:• Purchase price $150,000• Rent: $1,000/Mo. or $12,000/Yr.• Financing: 20% down, 4.5% interest, 30 year term.

I see a lot of projects in places like Indianapolis, Cincinnati, & Charlotte and there can be variations in costs, especially on the material side.  

I think this is a great new feature.http://www.biggerpockets.com/forums/25/topics/80292-bp-update-73-launch-of-the-biggerpockets-forums-keyword-alert-systemI've just started using it and have put in about 20 key word alerts.I've put in a number of cities that interest me, including Dallas and Austin.I've put in a few variations of SD-IRA searches.

A variation on the Nigerian scams?

Ceramic can crack in fixtures, may just repalce faucets.Slabs can crack with wide quick tempature variations, the heat is off keeping the floor at a more constant temp.Check the windows and doors, there is more expansion and contraction in frames without temps controlled and mositure removed.

Look at the numbers anyway and see if it makes much sense,SS in progress 120k [will go no higher]Closing, say 5kHOA fee, 250/month [master insurance and exterior]Taxes at 170/monthRental comp reliably at 1250-1350; relatively good market, will attract A class tenants.Investor mortgage 4% 10 yr balloon, amortized over 30yrs, P & I, 430/month.This unit would sell at 140k (+/-5k variation max) if marketed in the MLS.

There seems to be variation within a zipcode though.

You can use some variation of that but you get the picture.