Hi Guy's
Just thought I,ed show you a home made tool I use to square up and cut size my plastic card, First a stout bit of wood a off cut of kitchen work top is Ideal, I like to cover mine with some cutting mat then two steel rules set at 90 degrees with short one on top of the longer one ( I have two sizes 450mm x 300 plus a 900mm x 600mm ) a washer under the end of the shorter one which leaves a 1mm gap under it.
I also use a single edge knife to mark my cut as a pencil line at 1:72 scale works out at about 6mm, I hope this may be of help to some of you.
Regards Howard