I would typically look at historical collections and then make an estimate for future income. 

For estimating renovation costs one would need something more detailed than a flipping calculator since dozens of little items can add up to big budget overruns.

Prior to purchase 2 years ago, I had determined that a reasonable rent was $700-850, with the caveat that these are a bit hard to estimate as 3-bedroom units in a 2-bedroom market, with the units a bit more upscale that typical in the neighborhood.

Equity doesn't really exist until you sell the property - until then it's an estimate of equity.  

So are y'all telling me that the roofer should have been able to tell me exactly how many roofs were on there when he did his estimate?  

When you check that ARV, check your repair estimate.

Thanks everyone...I am purchasing 8 units in a building of 40...my current estimate is 2500 annually which is not an impact on cash flow.

In doing so, I found a document in the calculator section titled "Estimating ARV Primer".

Quick and dirty analysisARV - 58000Purchase price: 14,900Closing; $1000Holding cost (utilities): $400Taxes (200)Rehab; 30,000 Selling costs: (6%) 3,460Closing Asst. (1%) 580Potential Profit = $7460Rough estimate - you have to be very strict on your rehab cost - a 5k overrun will potentially kill this deal.

It's difficult to make a quantitative comparison, as there are so many variables involved.However- I'm editing the first post- I just found a site with estimates for heating costs according to the type of heat used.