The "Default" outbound policy rule of "Auto" uses a simple weighted balance algorithm that is based on the downstream bandwidth setting of each individual WAN:
This should be set to the actual speed of the line on the WAN setup page:
In my case, WAN1 is 50Mbps and WAN2 is 100Mbps. The "Auto" rule will send 2/3 of the traffic out WAN2 and the other 1/3 out WAN1. If I go to whatsmyip three times, two of them will show that I am coming from WAN2 and one will show WAN1.
Of course if you don't want to use the "Auto" rule, you can simply change it to "Custom" and make your own default rule.
Hope this helps....