I think they just provide marks for the more common bends. You would usually use around 10° for little kicks anyway.
If you need too, you could make your own 15° mark by using a scrap piece of EMT, a protractor and a file to make a mark level with the floor.
Then color in the mark with a black sharpie to make it easier to see.
Speaking of Gardner Bender, the thing that takes some getting use to with those is that the when the handle is straight up it’s a 45° bend, as opposed to a 30° bend for Benfield style benders. So, your multiplier is 1.4 with 3/8” shrink per inch verses 2.0 and 1/4” shrink.
I say, if you want to do math all day use Gardner Bender, if you want to get some work done use Benfield.