Once you get the easel blades to be exactly where you want them, put a sheet of blank paper in the easel and trace the edges with a pencil.
The back of a print to be discarded works great.
Then label the result, and don't lose it.
You can then use that to re-set the easel each time.
I print the square on the rectangular paper, then trim off the unexposed portion of the paper when I mount the print on Mat board. The size and shape of the mat board will be dictated by the print.
Though this is designed for cutting mats, the principles involved are the same. I ALWAYS use this guide for presenting prints whether just on paper, or in mats.
Thanks John, that's a great resource.
I tried the calculations in the link (I've seen the same one in Way Beyond Monochrome) for printing 14x14cm on a 18x24 cm paper, and it turned out that I've the same vertical alignment as the calculation recommends :)
I went for 14x14cm with the paper in a portrait orientation, and I like the results very much.
I print something like Attachment 86470
Recently fotoimpex started to sell MCC 110 in 30x30cm and 40x40cm. May be this helps.