What is the formula for a 90 degree bend?

The formula for a 90 degree bend is stub length minus take-up. Measure the stub height you need, subtract the take-up for your conduit size, mark the pipe there, and put the mark at the bender's arrow.

Take-up is 5 inches for half inch EMT, 6 for three quarter, 8 for 1 inch. So a 12 inch stub in half inch EMT gets a mark at 7 inches.

Keep foot pressure on the shoe and bend smoothly to the 90 mark on the bender head. Air bending past the mark springs back a few degrees, so most benders build that in.

For back-to-back 90s, measure between marks and use the star point instead.

For offsets and saddles on the same run, the saddle bend calculator gives the marks and multipliers without the memorized tables.