Chapter 1 What’s New for FortiOS 5.2.1 : System Features : Using a Virtual WAN Link for Redundant Internet Connections : Measured Volume Based Distribution
  
Measured Volume Based Distribution
Your FortiGate can actively measure the volume of traffic sent to each WAN link and distribute new sessions to balance the traffic volume to each link using a simple ratio calculation. To use this feature, use the following CLI command:
config system virtual-wan-link
set load-balance-mode measured-volume-based
config member
edit <ID>
set volume-ratio <number>
end
end
end