Yeah, that's a basic area comparison, but for flow, it's going to behave differently.
Check out
www.lmnoeng.com and look up the calculation for this type of flow. Oh heck... (searching)
Here's a link to it:
http://www.lmnoeng.com/PipeDuct.htm
As Dave pointed out, there's a point where the opening is so narrow, that it doesn't matter how far it extends.