The overlap will not change your ratio, assuming you have a even pattern across the array. Your ratio is determined by the number of each LED that you have, really you WANT them to mix well. If they dont you end up with color separation and you can get some really funky multi-colored shadows in your tank. Obviously if you put all your blues on one side and all your whites on the other it wont be even and you will basically have a different ratio across the tank.
You are right about your flashlight on the wall moving back getting dimmer and dimmer. It is EXACTLY what is happening here, but the fact of the matter is at the distances we are talking about you are still getting plenty of light in the tank. ALSO, think about your flashlight analogy with a light you can focus, such as a mag light. when you are a foot away from the wall it doesnt really matter where you have the focus adjusted, you get pretty much the same brightness. As you move back to 5 feet from the wall you will have to focus it tighter to get the same brightness as before. Move back to 10 feet and you have to make it even tighter. same goes for the LED's, except we are using multiple light sources (individual led's) which is where the spotlighting comes into play with a tight lense at a short distance from the water. If you put all 20 of your leds, really close together you wont get spotlighting of individual LED's with say a tight 45deg lense, but you also wont cover your whole tank, and you will probably toast anything under those lights. You spread them apart more and use a wider lense to get more coverage. YES the light is less intense for the same number of LEDs but you will NOT be able to run them at full power with 45 deg lenses anyway. (and FWIW you probably wont be running them at full power with 60 or 80 deg lenses either)
I hope that all made sense. You are right in your thought that you will harness more of your light into the tank with a tighter lense, and I know that is why you keep wanting to go with a tigher lense. But in reality, you will be getting PLENTY of light into the tank with 60 or 80 deg lenses at under 12" from the water, and it will be more tunable to make your corals happy and not just burn them. Some people even run NO lenses on their systems at <10" from the water.