The circumference of a circle is 2*pi*r (2 x 3.1415 x radius). There are 360 degrees in a circle, so just take the ratio- 90 degrees would be 1/4 of the circumference. You just have to know the radius of your bender. Be aware that the effective radius changes depending on what size conduit you're bending, since the inner radius will always be the same for your bender, but you need to use the mid-point radius for this calculation.
If it's a REALLY complicated bend, I'll just draw it in CAD and measure the arc-lengths there instead of trying to calculate it all by hand.
