Estimating rents by mortgage/LTV

[Robert Scaife]

This may be a strange question, but what's an easy way to estimate a good rent based on a simple mortgage amount? Is there an easy, "do in your head" way to estimate w/o pulling out a calculator?   I found a property I liked in my hometown and it is in a great location, close to the school and the park (within 250m either was), but when I plugged it into the rental calc I was getting strange numbers like a 1.15 on the 2% rule... etc. I was estimating rent on the market, so perhaps the property is over-valued for a rental?

Anyway, I hope my question makes sense.