I've found that most of the time when I do a chamfer I have the length of the chamfer and not distance back I want to trim. If you add this line into your mnu file you can have AutoCAD figure the trim distance for you.
[Chamf leng]^C^Ccal;c=\chamfer;d;'cal;(c/2)/sin(ang(int,nea,nea)/2);;;It works off of a simple trig formula for an isosceles triangle. You give it side "c" and angle "C" it figures out sides "a" and "b" since a=b. This is assuming that angles "A" and "B" are equal. You must pick the angle in a counter clock wise direction.
