I generally eyeball the box offsets, and use the "Benfield" method for the rest.
I've used the "bender handle" method. Bend the first half of the offset, then lay a straight piece of pipe (a bender handle without the belled end of the newer factory handles works fine) parallel against the conduit you're bending, on the opposite side of the bend. Measure your wanted offset depth at a right angle from the inside edge of the straight piece to the conduit past the first kick. Your tape measure will be the "c" leg in the right triangle of the straight piece, the conduit and the tape. Where your wanted measurement meets the kicked conduit, that's your mark. The right angle part is important, too.
Clear as mud?
