Majority of my sites receive top rankings for the keywords in the SERPS, although may not be receive higher page ranks.

When submitting to directories you need to ask yourself what you want to do and depends on your situation. Initial sites with page rank 0 and are not listed for example may want to get indexed fast, and build their page rank. We'll being listed 20 links deep in a directory (Like Page 20) is not going to build you page rank, but will get your sites indexed and keep the search engines coming.

If you want to build your page rank higher, drive more traffic, and have more effective targeted back links, and become more important to the search engines, you need to be listed in the Top Level Pages (Listed between 1 - 3 Level Deep within the directory). You can do this by purchasing upgrades, featured ad links, or top listing in your specific category or by already have a very high page ranked site.

A lot of folks will find my directories very useful and beneficial to their SEO and traffic building goals regardless of my sites page rank by having their sites listed in a directory that is SEO Friendly, high trafficked directory that produces quality traffic to their sites, keeping their sites indexed and remaining in the top SERPS for their targeted keywords, building up their Page Rank through Featured Ad links, etc...

My Page Rank Formula, in my opinion, Page Ranks are built from having several targeted keyworded back links from sites that are ranked well in Google, that receive large amount of traffic, and having your site listed on the Top Level pages or better yet on the main page.