AHHH XD it took me half a year how to figure that.wonderful question my friend!i do not use a 3d program, but i do use a piece yarn/sewing thread/(sometimes my headphones cable)etc.when i`m tracing the pattern in a vector program XD(this will sound ridiculous) i zoom out enough to see both parts, and in real life i put the tread carefully against the screen following the shape i`m interested in, then i measure it with a ruler or compare it putting it over the other part(the one that has to measure like the other and make adjustments as needed). LOL i dunno if i explained it clear i hope you understand.other times i directly create in the vector program a curve just as the one that needs measuring over it and jut rotate it with the center on the beginning, like if i were using a compass and opening it.