Short usage info:
In loopcut mode after selecting a 'reference edge' and you are selecting the % to cut press p to toggle non-proportional mode.
the base side of the reference edge will be indicated by a large pink vertex. This is the side that 0% is calculated from and the side that the non-proportional cut will mimic. To change the side of the base, press the Fkey
The header has always shown the % of the cut, but now in non-proportional mode, it will show the distance from the base side of the loop. It will also show which cut mode you are currently in.
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