Using UHF CCR's at 2 watts we can get 3ish miles in the mtns.
As for transmitted signal strength vs distance, here is the best way I know to splain it.
Remember the old CB's with the analog meters? If you are hearing me at a 5 on the meter, I would have to 4x my output to put a 6 on your meter. To make it a 7, it is another 4x. So in the above example, 4 watt was 5db, 16 watts made 6db, 64 watts pushed it to 7db. Now if I want to make it 8db I'll need to transmit at 256 watts.
Does that make sense? hope so. I really am a poor teacher....
Can I suggest that you do some homework on the numbers your throwing around.
An example of what you should be saying is if you start out with 2 watts, you need to double the power to 4 watts to get an additional 3 db. The next step would be 8 watts for an additional 3 db bringing you up to a plus 6 db. Going from 8 watts to 16 watts, you get another 3 db gain. That now has you at a plus 9 db over the original 2 watts.