I think you can scale everything off of the distance from the bottom of your screen to center of reticle in-game. So if your distance is 6 inches, then the scaling is 6/5.3125=1.1294. Then set your printer scaling to 1.1294, and it should print to your scale. I think.
As for how it works, yes, you do need more than just an overlay. This overlay only gets you close. It only shows where the round will burst. You need practice and experience in figuring the flight path and lead.
On another note, I used the overlay on the tank island map. Even though you can't see the corner airfields from the gun emplacements at the outermost vehicle fields, I was able to hit the runway and radar with AP rounds at 8,650 yards. I knew I was getting hits because you can see a sprite when a round hits a structure or vehicle, even at distance. GYK or KrisKros (can't remember) was AFK on a hill in a jeep halfway to the base, so hopped in a gun and got some practice in. I was able load the film and see what the sprites were from. I was hitting the bomber hangar from almost 5 miles away.