Are the Rent numbers of Rent-O-Meter accurate for Chicago area?

[David Van Brunt]

Just getting started, and looking to buy first rental property an Norther suburbs of Lake County. Given high property taxes, my calculations are very sensitive to rent assumptions. Have check numbers on Rent-O-Meter and RentFax, which tend to converge around a particular figure. However, If that number is off by a few hundred a month, the single family residence goes from 15% CoC return to nearly 10-- and 12 is my cutoff.

1- in your experience, how good are those estimator service numbers?

2- are there other sources that are more reliable?

3- are my return expectations unreasonable for the area?