Proportioning Hops for Flavor

I've done a dozen batches now, and for the last few I've been developing my own recipes. Rewarding and fun, but I am also looking for predictability.

I certainly understand the standard hops additions as far as bittering, flavoring and aroma, and the associated timing- However, I would like to get a better sense of how to proportion flavoring hops so that the desired flavor comes through. Is there a certain percentage/rule of thumb of IBU's that folks designate for their flavoring hops? Does an IBU at 30 minutes count more than an IBU at 10 minutes (my current logic)? The best info I found so far it this Mr. Malty article about late hops additions-

http://www.mrmalty.com/late_hopping.php

I'm looking for any other information or input. Thanks.

I wrote a blog post on just that topic:
http://woodlandbrew.blogspot.com/2012/11/how-much-hops-is-enough.html

There is also a link from the blog to another good resource.

(Don't worry, my book is writen much better than my blog)