When it was new, I believe it came with a small spacer that was inserted into the left-hand side of the slot... and you placed the paper against that "filler" for centered prints.
Ken
Actually, Saunders design has three slots - one for 5x7 paper, one for 8x10 paper, and one for 11x14 paper. The filler was used to center paper for making 4x5" prints using the 5x7" paper slot.
A way of calibrating your process involves using a defective print (don't tell me that all of your prints are perfect!). After it has dried, flip it over to work on the back. Load it into your easel. with the paper moved as far to the left as it will go in the slot. Then, adjust the masks using the scales on the sides, and then use them to draw a line on the back of the print where the border will be. Take the print out of the easel and inspect the border. If it is even, you are golden. But if it is off, you can measure how much you need to shift the borders, and in which direction. Then, you can use shift to bias your border adjustments. You probably need to repeat the process for each of the slots in your easel.
But you also should recognize that the dimensions of paper is not always exact, and if you purchase a new pack of paper, you may find that it is slightly larger or smaller in one dimension, thereby confounding your calibration.
I have a related problem - I routinely cut 11x14 paper into two 7x10" sheets plus a couple of test strips. Then, when I use the 8x10" slot in the easel, I have a centering problem.
I've concluded that the answer is to do the best that I can, and then be prepared to trim the print after it has been processed and dried. The various steps involved in processing frequently cause the edges of the paper to be abused, and trimming has the advantage of also restoring a clean edge.