Get a big printing easel, that'll make life easy. With that you can project your image down onto a test sheet of paper (or use a red filter) and move the easel parts around until you crop into the image very slightly. The easel should easily give you a ~1 inch white border. You'll need to cut the paper after drying to get a consistent border but you won't lose too much paper. If you are clever then you will pre-cut your paper and use the [unexposed] leftovers as test strips.
35mm ratio is a pain in the butt IMHO. You really need an easel to deal with it.
An alternative, if you like the look, is to file out your neg holder's edges so that you see sprocket holes and all. Some people prefer that to white borders. In the digi-age, it's now regarded as a novelty!