How to create plans

Hi guys. How are you creating your plans?
My confussion is  related to disk space and total email quota.
If I create a plan, and assign a total mailbox quota of 1Gb, 100 POP3 accounts, and 3 Gb for disk space .. then common sense tells me the user has 2 Gb for website/db/logs.
But what if a user creates only 2 POP3 of 100 Mb each? He can use up to 2.8 Gb for webspace.. Is that correct?
And even if he creates only 2 email accounts of 100Mb each, but only uses 2 Mb in each account... that would count 4 MB email... and 2.96 Gb for web space. Am I correct?
Thanks for your time, I'm a bit confused here.