Read ThrustHP's help documentation (included with the program). It has a comment about pitch. The forumula's it uses were published in AMA magazine in the 80's.

I've been using the spreadsheet below recently, it seems to be more reasonable, and the thrust estimate varies with pitch.
http://home.wanadoo.nl/pereivers/Goodies.html
Download the "prop-power.xls" spread sheet.