I tend to just use my finger(s) between the knife and frame as a spacer. Might not be the safest, but I find it easier to adjust for width and faster than reaching for another tool.
I'm trying to picture the washer but can't lol. Could you post a picture sometime?when i have window perf i cut it 1 of 2 ways. a good tuck and edge cut or i have a little metal washer i put the knife into and it glides along making a uniform cut