I agree with Tom, but would add that it's only reliable if you're sure of the current end of the roll.
If the last cut made off the roll was at a slight angle, you'll end up with a slight rhombus instead of a square. If you can stand to waste a little of the roll, I would recommend rolling out (but not cutting) around twice what you think you'll need and folding the paper back along itself, being sure to give yourself slightly more than a square's worth. This way, you can align the long edges of the roll against themselves, ensuring that fold line is perpendicular to them. Then, use Tom's approach, bisecting one of the corners that falls on the new fold line. You'll likely end up with a strip of excess hanging off the edge of the corner that you'll need to cut off, but if the starting edge wasn't true, it'll be better to waste that small strip than all your efforts. Best of luck with your ambitious project!
