doubt Flow Monitor

You can select WAN to view your WAN traffic, but you may also see a lot of other traffic that the firewall is probably blocking.  In other words, don't let what you see surprise you.

One more question,

In saturation problems should I be guided by the LAN or WAN interface?

Very Thanks !!!

It depends on what you are trying to find out.  If you think a user is saturating the network connection, then I typically use the LAN (especially if you only have 2 interfaces).  If you have more than two interfaces, or if you are trying to find out what they are talking to, I would use WAN.  I have three interfaces, but still use LAN mostly because I don't have a lot of traffic between the two internal interfaces so what I see on LAN is mostly internet-based.