For embossed plasticard I measure, and then practise my cut with a pencil. That way I can be sure that the knife is likely to go in the right place.
Embossed plasticard can be a pig to cut. The very lumps and bumps we want it for will cause the blade to jump around if you don't cut very lightly for the first few passes. Again, it's a whole lot easier if you have a pencil line to align the steel ruler with. And don't try to do it freehand, except perhaps if you are cutting along the line of brick courses.
And even then a pencil line is a a good idea. If I don't use one I get lost half way along the sheet. You might think that it's difficult to get lost working ins straight line but when truing up the bottom of a sheet, I've managed it. Maybe I'm just special !
Cutting board sale update: We have a bidder ! And 5 people watching. Looks good so far. Go and have a look for yourself.