Welcome to the Quilting Board!
Most of the quilts I make, the measurements need to be consistant so which ever way I start measuring, I continue throughout the quilt.
I definitely would measure my mat and if it is true - use it.
My sister told me about going to her local Lowes store and having them cut her the size squares, rectangles etc. from the plexiglass they have in the store. Very inexpensive to do. However, you must stress how important it is that they be perfectly square. One drawback is they don't have the lines for measuring. Whatever size you need, just be sure you add the seam allowance. Hope this helps.
I love my strip cut ruler!
Once you get your measurement with your smaller ruler, you can butt a longer construction level up against it to use as your straight edge, or a yard stick...or any other clean straight edge just so you have the longer length for the cut. DH gave me an AWESOME heavy duty straight edge- it's either 48" or 60" long...not sure where he got it...but it's perfect for longer cuts. Also snagged his metal construction square, which also comes in very handy.
The straight edge is heavy enough that it doesn't require a lot of pressure to hold it steady to make the long cut.
Last edited by Teeler; 04-25-2013 at 10:57 AM.