[Asphyx]

For Verizon set it to 12...
For the others I think 24 will be more than enough...

For some reason Verizon had HTC dedicate some of the memory to the radio instead.

I'm running MightyMike's rom on a Verizon TP with a PP of 12.
Runs very smoothly. I also disable MS Voice Command (never use it with Manila) to get me a little more program memory...
I hover between 55% and 75% free ram depending on how long ago I reset.

There is also a good memory clear cab
Search for HTC Addicts CleanRam...(Try Xda if it's not posted here)
Will clear about 6-8 Mb of ram out for when you need space and don't want to reset the phone!