Mounting can be a can of worms. Back at Ohio University in my Hippy days, I was taught to mount above center but never heard the words 1/3rd - 2/3rds. I have often mounted prints, square and not square on boards 2 or 3 sizes up from the long size of the print, i.e. 4x5 on an 11x14 per say.

I usually put the print image down to protect it on the tissue, use a mechanical pencil without lead to score the tissue real size to the print. I then use a straight edge and razor to cut just inside the line by an 1/8 of an inch or so on all sides. I find some tissues grow in the hot press. I tack the middle then set aside and repeat until all prints have tacked tissue waiting for the layout and heat press.

Then I find where I want to mount them and mark on the stainless steel T square the tops of the print and bottom for where it goes on the board, slide the T square left to right to see if the image is level. I use a second ruller maked for the left to right. If they are all the same I can wizz through a lot of them very quickly.

3 corner tack all the prints to the board then use a cover board in the heat press. So I would batch your 10x10, 15x15 and 8x8 to speed up and be consistant.

Pre-cut mattes are not too bad to match to the mount boards. 4 small light pensil dots of the frame onto the mount board gives you measurements you can use to layout your backing board. So just make certain you keep the overmat matched to the mount board once the print is tacked, else you may find out that not all matts are square and the same.