Nobody would say that a 24" ruler is the same as a 12" ruler however when it comes to optics people say that they are the same all the time. Measure the distance from your shooting eye to your front sight when fully extended. Divide that in to 40.
Example : say it measures 30"s. 40/30 =1.3 so a + 1.25 reader will give you very good focus on your front sight. However it will blurr your distance. Now if you you are nearsighted -1.25 diopters then you are already in focus at about 30"s tell me what your distance correction with your shooting eye is and I can tell you the power that will work for you.