Hi asgsoft,
The best way to determine if an expiring domain will keep it's PageRank is to check:
DigPageRank Pagerank Checker Tool in over 700 datacenters the day it expires. If there are any datacenters showing green valid PR, then it has a 99% of keeping it's PR if developed & taken seriously.
If the day it drops, it is showing fraud or "unknown" PR, it is lost & useless to register the domain.
Google has a quarter update (every 3 months). But what many fail to realize is that they also have a weekly update (between Thursday & up to Saturday night every week) USA time. Your best bet would be to catch a drop on Monday & develop it by Monday night. Then when google scrapes it's weekly PR (on Thursday), you get to keep your PR since the site is developed.
It gets tricky & very risky with drops that become available / you catch between Thursday & Saturday. It will require a lot of digpagerank.com monitoring & super fast development. But if google scrapes your domain on their weekly visit, and your domain gives off bad status page codes like: 404 (page not found), 302 (redirect) etc. then the PR will be instantly lost. It must remain Status code 200 (status ok) and have some related content on the main page at least.
Sorry if this is technical for some, just sharing my tactics & I have been very successful at catching & flipping high PR domains Even PR6 & 7's that I grabbed at regfee. It is much harder now & I would be lucky to find 1-3 PR4 domains a week vs the 100+ PR4's I was grabbing a day when domain tasting was in effect.