It’s not exactly what they teach in letterpress school, but I like to use graph paper. It provides horizontal and vertical lines that are so helpful when I’m registering. I have tympan paper laying around, but graph paper (with proper packing) seems to works well for me. I use a nice clean sheet every time. It’s very inexpensive and it goes into the recycle bin when I’m done with each job.I’m actually curious about using Mylar. Graph paper works, so I haven’t picked any up yet.