Do it by eye or just get the angle with the rule you need - handy for underfloor angles etc
Lay the rule on the floor and pull to that angle looking down the bender.
Much easier to bend it using a frame bender. Hand benders are much harder to use because they are moving as you hold them.
As said, do the offset first and then line in up for doing the 90 degree
Get some practice in on some scrap. I try to use the hand bender as much as possible, even when it cant be seen. Once had 5 bends and a set on one full length of copper that was going under floor in an extension, had to tweak it a bit when fitting but looked awesome and gaffer was well impressed!