Can you provide additional details on your process for mounting 17 x 30 inch prints? Made up some 17 x 24's and got close (1/16 inch) but would like to be dead on. i.e. you only get one shot at getting it correct.
Perhaps a photo of how you use the standoff would be appreciated so that the prints exactly match the mounting board.
Since it's foamcore I'm mounting to, I don't care if it's a little bit out of alignment. I can easily trim it after mounting. I get pretty darn close, though, with my technique. I use an Olfa utility knife with snap-off fresh, sharp blades. If you're mounting on masonite, ymmv.
Take a few, narrow strips of foamcore or anything slender and non sticky (soda straws work well, but they're too short for 17" wide prints) and lay them on the upper surface of the previously-coated substrate. Position the print carefully, laying it on top of the standoffs, but keeping it just off the surface of the substrate. The thinner your standoffs, the closer the print can be to the surface and the easier it is to verify alignment. However, you need more of the thin standoffs to prohibit contact.
Once you have good alignment, withdraw the centermost standoff and allow the print to contact the substrate from edge to edge, smoothing it onto the surface with your (gloved) hands or a clean cloth. Then, working outwards left and right from the center, gradually apply the print to the substrate, smoothing and adhering, removing standoffs as you go. I screwed up the first one, but learned from my mistakes. I suggest a dry run with an unimportant print first.
I'm very satisfied with the results, and I've done a fair bit of dry mounting with a proper press. Next thing I need to figure out is how to make the edges of the foamcore black.