[CentOS] question(s) about LVM snapshots

I've recently begun to try out LVM snapshots and I have 2 things I'm  
wondering about

My intention is to run backups off the snapshot. As a first  
iteration, I left 15% of the disk for the snapshot LV.

1.
The HOWTO instructions suggest that the snapshot is in effect as long  
as the snapshot LV exists, and use lvremove to disable the snapshot.   
Is this the proper way?

2.
I chose 15% because the disk  is not that active and I think it will  
be plenty for the duration of the backup I'm doing. Is there a way to  
track how much space the snapshot is actually using?

Cheers,

Tony Schreiner
Biology Department
Boston College