Yes you would subtract the risk free rate if trying finding the equity risk premium. If you’re using Ibbotsen Chen for required return on equity, you would not subtract the risk free rate.
It wont be plug and chug - but could be more theorically based. Also, they could trip you up making you calc the expected earnings growth estimate (ie: give you 2016 and 2017 results and make you infer what occured) and what that does in terms of the ERP within the Ibbotsen Chen formula. just my 2cents