Why not add an on-the-fly overscan tool?
Setting the value manually in /boot/config.txt isn’t really pragmatic. Lot’s of trial and error. I found a tool here: https://github.com/ukscone/set_overscan
But that didn’t work for me. The screen that appared didn’t change size. So I didn’t see the effect of the new setings. And the new settings were addade to the /boot/config.txt instead of changing the settings, which means that your config.txt will grow each time you use the tools. Of course that is an easy fix, but since the setup screen didn’t change size anyway….. maybe the tool just isn’t RPi2 compatible, I didn’t try is on a RPi1.
Anyway … a working version would be very neat for Retropie, especially for people who take their gaming computer with them an plug it in another screen regularly. Kodi has a nice calibration screen that works on the fly.
It would be nice to extract that bit of code for a retropie overscan calibration tool.