I have been doing some FM DXing lately, and have been trying to come
up with some "number" to give a relative value to what I have heard.
I know that I cannot look at just transmitter power and distance,
because the transmitter's HAAT has a definite bearing (pardon the
pun...not a reference to a directional antenna). Do any of you know
of a website (or anything else) that can clearly specify the important
criteria for this kind of evaluation, and maybe supply equations as
well?

I have studied the FCC "FM Curves" and other information they have
posted, and the closest thing I can find is a page that specifies the
maximum ERP for a station given the class of a station and the antenna
HAAT. I don't honestly know if anything that I can derive from that
would be directly applicable to what I'm trying to do. I also may
have my head firmly lodged somewhere thinking that this can be done
too. Advice and suggestions gladly accepted.