My way of setting the size of the rear loop is always the same. I let the housing tell me. I put a ferrule (if using one) on the RD end, and put it into the fitting. Then I hold the other end along the line of the frame fitting and advance or withdraw housing until the RD end lines up with the fitting. That's the best length since it prevents an angle where the wire emerges.

Sometimes, however, I can't get natural alignment at both ends, so I have to cheat. I might slip the housing into a piece of tubing and curve it into a sharper bend, or sometimes I can set the sharper bend by working the housing in my fingers. If there's a practical place to tie the loop to force it to the line I want, I'll do that. (I'll often resort to a zip-tie on the stay about an inch out from the stop for this. On my commeter, I trap the housing below the QR nut which makes a good line for the chainstay, and a clean tight radius bend for the RD end.

In short there's no single best way. The object is good alignment at the ends, but how to do that is up to you and will vary bike to bike.