So it seems that I'm stuck with this simple challenge:
Every first day of the month, our system records the number of members for the previous month and records it as a table (see example below).
I would like to be able to calculate the difference between the most recent period (highlighted in green) vs the oldest period (highlighted in yellow).
Using the table below as an example, the result should be 20 (and I would like to call it "Acquisition Rate").
There's no workflow at this stage, the table below is coming from my DB as it is.
Any ideas?

TIA