Referring to the Camera Re-calibration page, instructions say to put a 20 X 12 inch Proofgrade material on the bed of the Glowforge, barcode side down.
The actual size of my Proofgrade acrylic is 20+ 7/16 X 11+13/16.
My Proofgrade medium draftboard size is 20+7/16 X 12 +1/16.
My question is how will the actual size of the material used which is not 20 X 12 inches affect the recalibration?
Just use the draftboard pushed tight against the bottom right of the crumb tray, If you use a couple layers of white masking on it first so much the better as the scoring might not go through.
You can use any material and size does not matter as long as it covers the printable area of the bed.
I wasted my remaining piece of draftboard, then used two pieces of chipboard taped together from the back, then a piece of cardboard from a large shipping box, then the same with a sheet of 12" masking over it. (I had to attempt it many times, I think about a dozen, before it worked for me.)
The size of the Proofgrade material you mentioned will be enough to cover the printable area of the Glowforge, and this is just fine to proceed with the camera recalibrator.
If you run into any other trouble, please start a new thread or email us at support@glowforge.com.