I hear that if a site has one PR(N) backlink, then that site most like get a PR(N-1) on the next update. ei: your site got a PR6 backlink, your site is PR5. Is this true?
no its not really true to be honest PR is a strange thing Some of my sites have thousands of backlinks and its only PR1 whilst others have only a few very relevant ones which seems to perform better
It is possible in theory according to the google PR formula published in wikipedia. But That pr6 page should not have any other outbound links except yours.
You can't work out PR unless you work for Google because various factors fluctuate all the time. For example your toolbar might show PR6 but in reality its PR1 because visual PR has not updated yet. Same thing with backlinks and other things.
Just continue to build more backlinks, and you'll get a higher PR, but mainly you'll get higher search engine rankings.