No formula needed. Just find something that has a true right angle like say a square box cover or a door frame. Make one mark from the corner the first dimension and make the second mark from the corner the second dimension. Measure across from mark to mark with a ruler and that's the roll length or you can use the Pythagorean theorem if you have a calculator that can do square root.
An accounting of the runs should be done! For conversation let's go with 1 1/2"space on run of conduits - 2" past any obstruction. Your minimum past distance will never change; nor will anything else; there is no angle gain in a continual set of offsets, just the placement of the next conduit from the spacing.