Auto steps/mm correction
I guess that this would be easy function for you. To add a g code command where it would go from min to max Endstop and compare the length it think it is to the real length written in EEPROM. Then it could automatically fill in the right steps per mm or write them for user to type in.
Comments
Better solution is to print 2 blocks 5 and 10cm. Take the difference in length which equals 50mm planned. Correct stepsnew = stepsOld * 50/distanceMeasured and store it in eeprom using eeprom editor.
Difference because settings can make objects slightly bigger but both blocks should have the same error so you subtract error of same size and get real width.
That is a feature nearly no one would even notice exists and use. So it would be very low in my priority list. So do not expect to see it.