If you use Fireware XTM with a Pro upgrade, you can assign a weight to each interface used in your round-robin multi-WAN configuration. By default, each interface has a weight of 1. The weight refers to the proportion of load that the XTM device sends through an interface.
You can use only whole numbers for the interface weights; no fractions or decimals are allowed. For optimal load balancing, you might have to do a calculation to know the whole-number weight to assign for each interface. Use a common multiplier so that the relative proportion of the bandwidth given by each external connection is resolved to whole numbers.
For example, suppose you have three Internet connections. One ISP gives you 6 Mbps, another ISP gives you 1.5 Mbps, and a third gives you 768 Kbps. Convert the proportion to whole numbers:
The results are 8, 2, and 1. You could use these numbers as weights in a round-robin multi-WAN configuration.