This is going to be used in my port length calculator. I'm not looking for an exact number, just general figures, like a range, for large, medium, and smaller woofers.

Since magnet size has a very direct effecton this, would using that as a marker for the user be ok?
Example :
user sees the form, it asks what size magnet his subs have. "ok, i have 90 oz magnets, I select this one" and from there I can get a general idea of how much his sub will displace when mounted.