Rotatrim Question

Hi, brand new here, and just getting started with an Epson 3880 and some printing.

I had a quick question about Rotatrim trimmers.....

I am considering buying the 18" version, but my question is, how do you ever get accurate cuts on sheets of paper that are more than 12" long? In other words, if I start with an 11" x 17" sheet of paper and want to make an 11" x 14" print, how do I line it up so that's it's not going 2" off of the board/ruler, since the other dimension is 12"? (not the blade side, the side perpendicular to that).

Maybe I'm missing something?

Thank you so much for any help. Cheers.