User and Device Licenses for Small Business Server 2003

I'm still confused about what is the requirement for Small Business Server when purchasing licenses. I understand that a user CAL lets one user use several devices (PC, Laptop, PDA) and a device CAL lets any amount of users access? (20 users using 10 computers in shifts?)

Ok so we had our original 5 when the SBS was installed, then we purchased 5 more licenses and I wasn't around, I recently purchased 5 more device licenses (the packaging says Device Licenses. Ok; so this is our scenario right now: we have 18 computers,  usually 1 user per computer and maybe 3 floating people that log on to any one computer when the original user does not come in. recently when I had only 10 licenses some computers when I added a new user to it, that user or another veteran user would get disconnected from the server somehow and I was resolving this by basically restarting the computer. When I purchased the 5 devices cals that problem went  away. Now we are going to add 3 more computers and hire 5 more people (adding them as users on the server) so; my licensing screen on the server says I have 15 licenses and maximum usage 19 (it was 18 but went one up when I added another cell through ActiveSync). So what do I purchase now? User Cals or Device Cals?

And Part 2 of this question: How do I make sure that if I buy a 5 pack of BOTH user and device cals that it's the right item?