Don't get all caught up in this LED thing and trying to figure out all the math. First let me say LED's either work or they don't and they will do nothing to your module board that can damage it. Now I run G scale and I purchase mostly the 3mm, or the 5mm LED's on eBay with resistors, they are also rated at 12v on full power and I normally put the bright white in headlights and soft white in all other lights, interior, running lights, etc. So our RailPro modules are setup for 5v on the lights right, well guess what I run those 12v LED's on 5v and they burn just fine, yes the light is a little dimmer, but this gives me the light brightness that I want and if it's still to bright, I just add another 1K resistor. Many try to make the LED thing Rocket Science, which it's not, just buy in larger quantiles on eBay and test them out with the voltage you will be running, I use a old transformer and a test meter and set the dial with the meter to the voltage I want, I may even let the LED burn for 24 hours just to make sure it will work when installed in the engine, etc. I also do the same with any LED's regardless of size and voltage, just test them out, all they can do is go PUFF and smoke a little.
trainman