How to log traffic amount?

I have a bunch of RB2011UiAS-2HnD-IN, running RouterOS 6.31.

How can I log the amount of traffic going through a network interface with RouterOS 6.31?

Specifically, I’m about to set up a PC with SNMP, and a server will be running cacti to query the PC using SNMP. I want to know how much traffic is generated like this over a 2 hour period. This is because in the future, the RouterBoard and the SNMP querying will be running through OVPN over a 4G connection, and the 4G connection is quota limited per month.

I know RouterOS has a graphing feature, but there are two problems:

  1. It’s zoomed waaaay out;
  2. Even if I manage to zoom it in, I would still need to integrate (in the calculus sense) the graph myself.

routerboard graph.png
Are there better ways? I mean, I would like a simple way to answer a question like: how much traffic went through this interface from 10:00 AM to 12:00 PM today?

EDIT:
I just came across Traffic Flow (http://wiki.mikrotik.com/wiki/Manual:IP/Traffic_Flow). Would that be the right tool for my purpose?