At this point, I probably only have about 100' of track laid with only 6 pairs of track feeders. ... I can take voltage measurements anywhere on the layout and get the same reading.
Are you taking the measurements under load? If there is no load on the system then you will always get the same voltage measurements regardless of length.
Place a significant electrical load say an 1156 tail light bulb (= @15V 8 ohms 30W = 2 amps = 4 unit lash-up pulling really hard) on the rails at a feeder point. Measure the voltage at the bulb. Then place the same load 8' away (midway between feeders). Measure voltage at the bulb again. The difference in readings will show you the the effect of wide feeder spacing. Not saying it is a bad thing. Not saying it won't work. Just saying there will be a measurable difference. In the ideal world we want the loaded voltage to be exactly the same everywhere. Closely spaced feeders facilitate this. 16' is pretty wide spacing.
I did the same test on my system except under a load greater than I knew would ever be realistically expected on the rails at any one place. For a load I used an automotive headlight which draws about 4.5 amps @ 15V. The voltage measurements varied slightly from location to location but at worst were only 1.2 volts less than the unloaded power supply voltage of 15.1 volts. I now know that even with a lot of locos in a lash-up pulling really hard they will always receive at least 13.9 volts.
In both cases we are actually measuring the voltage drop of the entire bus and feeder system while under load. Regardless of wire gauge or number of feeders, this is the final answer that matters. It is what your locos will see.
The popular quarter test mentioned earlier is a test to determine if your wiring will allow enough current flow to trip the circuit breaker in the power supply. A good test. But it tells you nothing about what voltage the train will operate from under normal conditions. You really need to know both.
Why you ask? The breaker test is obvious - prevent damage. The voltage test because it feels really good to set the speed of a loco and watch it traverse the entire layout without one iota of speed change. Like your locos to creep at walking pace all the way around without stalling? Keep the voltage even. Ahhhhhh feels so good.
