Point calculations are based off of traffic passing through the FortiOS-VM interfaces. Points are used per terabyte of traffic and there is an increased point cost as you increase the FortiGuard services in use.
4 pts/TB for a FortiOS-VM tagged as "FW"
10 pts/TB for a FortiOS-VM tagged as "FW + URL"
25 pts/TB for a FortiOS-VM tagged as "UTM"
Reference: https://docs.fortinet.com/uploaded/files/4057/fortinetvm_on-demand_1.pdf