Estep starting distance

[agreen]

I normally mark my filament at 120mm, then re-measure it with calipers. It's often NOT what I marked - perfect example, I just did this and got 124.5mm from the extruder to the mark. This is easily solvable by allowing me to enter the starting length instead of just hard-coding it at 120mm. 120mm could even be the default value in this field, but still allow for when people can't do that exact distance for whatever reason.