http://www.thinkbling.com/prank/ I like the idea of this tool, but my only problem with it would be this: A PR 5 website shows "Google rates this site 5 out of 10" any non-savvy user will think that is a shotty rating.. lol
Simply say "Google ranks this page PR5". Have you tested your calculator with the results from Rustybricks new tool. I worked with Rustybrick's results quite a bit the other day and I found I got a perfect correlation with his results if I changed the assumption in my original chart from 50 links per page to 40. I have a new spreadsheet with the formulas that will forecast PR depending on the number of backlinks and their PR. If anyone wants to play with this spreadsheet let me know and I'll make it available.
Check out who is using it: http://www.thinkbling.com/prank/r/r.php (the number is the number of page views)