Unfortunately there is not an easy way to accomplish this. You will need to find the limit for each VM based on your testing or the VM size.
Keep in mind that the network bandwidth allocated to each virtual machine is metered on egress (outbound) traffic from the virtual machine. All network traffic leaving the virtual machine is counted toward the allocated limit, regardless of destination. For example, if a virtual machine has a 1,000-Mbps limit, that limit applies whether the outbound traffic is destined for another virtual machine in the same virtual network, or outside of Azure.
Base don this you could use something like this to calculate the percentage:
Network Utilization % = (Network Out Total in MB / Maximum Network Bandwidth in MB) * 100
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