Trying (and failing) to do "basic" maths in Grafana

Grafana with Influxdb

I am interested in graphing Disk Capacity (in MB) and Disk Usage (in MB) for Windows OS instances. Disk capacity is desired because most of the hosts I am interested in, are VMs, and the vDisk size may be expanded or shrunk during its lifetime, and I am interested in being able to see historic disk capacity and used capacity.

There are two winperf counters for each logical volume that telegraf sends to influxdb “Percent_Free_Space” and “Free_Megabytes”, unfortunately these are not quite what I am after, what I am after can be calculated. Capacity can be calculated with (Free_Megabytes / (Percent_Free_Space/100)), and Used Capacity can be calculated with (Capacity_MB - Free_Megabytes).

I am however completely at a loss as to how I can do this calculation in Grafana.