Is there any kind of tool that is a "block" (imagine a sanding block) that accepts utility-knife blades?
Imagine cutting a piece of rigid foam. And instead of using a straight pencil mark or (blue) snap line, and trying to follow the line with a utility knife.....
You could secure a straight edge (or a level with a bigger edge) 2" from the line, and the "block" would "ride" against the level/straight edge...and the blade being in the center of the block (but 2 inches from the edge) would cut a perfectly straight line.
Whenever I use a utility knife and the straight-edge/level, I still get a little wobbling.