Unfortunately I know of no similar trick for laying out the overmatte. Wish I did!
Okay, I'm glad you asked:
After the print is mounted, drop the overmat sheet onto it and align the top and right edges. Cut off just the tiniest tip of the upper right corner, which will give you an orientation reference. Now shift the mat down until its top edge is just below the top edge of the print (keeping the sides which meet at the clipped corner aligned) and make pencil tick marks at the left and right edges of the print.
Next, shift the print to the left, keeping the
top edges of the mat and print aligned, and make the corresponding tick marks. Rotate the assembly and repeat, keeping whichever edge joins the clipped corner aligned each time, until you have transferred the locations of all four corners of the print to the edges of the mat.
Now, using an accurate ruler (divisions to 1/16" or 1 mm, or better if possible) make offset marks that will give the desired window around the print. The actual offset amount will depend slightly on how the knife is set into the mat cutter, and needs to be adjusted with respect to the size of the print. (In my case, 1/4" top and sides and 3/8 bottom (to allow space for a signature on the print mount) looks right for an 8x10 in an 11x14 frame.) It is easy to get into the rhythm and make all four offsets the same, so double-check before proceeding...
Connect the offset marks with
very light, very fine pencil marks at each corner of the future opening; it is all right if they cross and overlap slightly (because you can erase them later) but you will need the intersection point to tell you when to stop with the mat cutter. It is surprisingly easy to make the marks intersect just at the corner, working only by eye, but it is not absolutely necessary if you have taken care to keep them erasable.
Finally, with the mat on a non-slip cutting board, put the point of the mat cutter blade through right at one corner (push at the correct angle if you are making the traditional beveled opening), bring a straightedge up to the cutter, and align the straightedge so that it is a precisely equal distance from the offset marks at the top and bottom. Make the cut, rotate the mat, and repeat three times, and you are done.
Now, wasn't that simple?
Using the same pair of edges for reference each time will ensure that any angular errors in the print location (from trimming or slippage while mounting, or lack of accuracy in the board edges) are precisely transferred to the location marks. A tiny error (say, 1/16 inch in 8 inches) is not so noticeable against the 1-1/2 or 2 inches between the image and frame, but really stands out in the quarter inch gap between the print edge and the edge of the mat opening. When hinging the mat to the mounting board, align the reference edges, and then split any residual errors (if you can even see them) when putting the assembly into the frame; the tick marks will disappear behind the frame rebate.