My latest design requires me to fold a narrow strip along two edges of the paper in order to find a few of the important reference points. (The ratio--strip to the rest of the square--is something like 1 : 4sqrt2+6.) Unfortunately, I need to use a really large sheet, and I find it next to impossible to fold parallel to the edge with only one reference point.

My question is this: are there any tricks to folding the strip accurately on a large sheet?

EDIT: Just to be clear, I know how to find the ratio. It's just that the required intersection is on one edge of the paper, and it wouldn't be of much use to mirror it to the opposite edge.