Waaayy too much work for me. That's why I use pre-cut overmats sized to float the print in the window. I don't have to measure anything. I don't have to cut any mat board. Well worth the money to have them cut for me.
Do you use the exact same printing dimensions every time?
Stop worrying about grain, resolution, sharpness, and everything else that doesn't have a damn thing to do with substance.