Bandwidth quota calculations

[#1]

Last month I hit my quota, for the first time in years and it turned out to be my son's game continually failing to try and download an update  The restrictions effectively made the connection unusable (remember those dial up days?),
So, because I run a firewall (ipcop) I have added some software that does bandwidth counting for me for 'all day' and separately for the 'peak' time.  It includes up and download amounts but since it has been running (4 days) +net's figures are consistently about 10% higher
Does anyone know the basis of +nets accounting? as I'd like to bring mine in line
--
Allan

[#2]

Could be a couple of reason for the difference. Your software could be using 1024 bytes per kilobyte where PN use 1000 bytes per kilobyte. Also your software will not record overheads only the data that get through the firewall.

[#3]

Well I suppose the '1000 bytes in a kilobyte' will make a small differnce, even if that descision is a little odd. 
I dont see what overheads there might be that could affect my bandwith comparisons. Every packet to the the router is sent to the firewall and before any drop/ignore is applied, it enters the accounting chain for recording.
It will be interesting to see if the percentage decreases if I use more bandwidth (ie it is an overhead/background traffic), so I'll try downloading some large files over the next few days