Thank you!
I mark out the edges with a permanent marker pen and hacksaw/padsaw close to the line. Then reapply more marker pen ink, and more often than not "scribe" a line
<purists look away now>
using the tips of the jaws of my electronic calipers, and file to the line
I try to use an existing straight edge on the blank sheet as a reference, or else I have to create one with the 12" rule and a scribed line.