That's very helpful! I didn't know you could specify a distance for an offset or shell that way... Here's another idea based on whay you have going already: simply let the user set a own Min/Max slider on the fly. Instead of the two clicks defining a set distance which is applied immediately, the two clicks define the Min (always 0) and the Max. Then as one slides between the two points, the offset, shell or fillet shrinks and grows.

Anyway, just a thought. Being able to pick a distance in the viewport definitely gets one in the ballpark for finer tuning with the keypad. Thanks!