I get a lot of mileage from a carpenter's square, along with a longer rigid measuring ruler. You square up one edge of the carpenter's square along the edge you trust, then bring up the longer ruler and make it flush with the square's edge that extends into the fabric. You can use this to mark out
points on a line that is parallel to the one you trust, and also to mark out
lines that are 90 degrees to the one you trust.
Well worth the investment ( < $10 at a Big Box place).