Continuing what @HarryC.Ragland said,
Once you find your current $102 value, you can just plug it into the formula:
10mm (expected travel) / 5mm (actual) * (current steps/mm) = new $102 value
Then just type $102 = abcd in the console and you should be good. You can then recalibrate to dial it in. Measure as accurately and precisely as you can.
P.S. I stumbled back in here. If you put a @ in front of my user name (as I did with Harry above), it will notify me.