Sometimes hard to fine tune the diopter adjustment on a
binocular by day for lack of an object with enough fine detail. Sure, a traffic sign or similar object can get you in the ball park, but I've found that the most precise way to set the diopter adjustment on a
binocular is to use a point source of light and nothing beats a
star when it comes to a point source. Just take your
binocular out at night under the stars and focus until that star is as close to a perfect pinpoint of light as possible. When both eyepieces show a perfect point of light, you're done. Not going to get the
diopter set any closer than that.