As you've suggested, it's dependent on usage patterns. I'll share my experience. My license is 15 Named User (NU) CALS and 15 Session CALs. Here's the usage data for a typical month (October 2009)
Most (~90%) of the sessions are concentrated bewtween 8-5. Mon-Fri.
# of distinct users: 165. 12 of the total were Named Users, the remaining 153 were Session users.
# of documents: ~100
# of doc sessions: Session CAL 2398, Named User CAL 1033, total 3431.
Peak concurrent Session CAL usage: 12 occurred once during the month, 10 occurred eight times.
Peak concurrent Named Users: 6 occurred once during the month, 5 occurred four times.
When a Session User has two documents open, that utilizes two Session CALs. A Named User can open many documents concurrently with the single Named User license. So I optimize my capacity by assigning the NUs to the heaviest users. You can see that by comparing the avg sessions for NUs (1033 / 12) vs the Session users (2398 / 153).
So in my experience, the 10 users per Session CAL suggestion is about right. I think you can get a few more than that with thoughtful distribution of NU licenses. QV9 complicates the NU assignment a bit, because assigning an NU is how you make someone a developer. I not on QV9 yet so I haven't yet tested the impact of that change on license usage.
Hope this helps.