It's taken me all morning, but I finally have an acceptable image size (been working on a 12x18 on 13x19 sheets). It's not perfect... for example, I can't put a stroke on the image because the stroke won't print on the tail end of the image. I had to create a custom page size of 13x20", no margins in the page setup area, but with a right margin of 1.025" in Lightroom. If I go to even 1.05", the image isn't centred anymore and any less of a right margin the image is cut off.
This is very frustrating. You should be able to select 13x19 sheets and print a 12x18 image perfectly centred, should't you?!!