In order to build out formulas that reference other cells, offsets are used in Finmark.
You can use square brackets at the end of the variable to offset months i.e. Suffix a variable name with [X] to refer to the Xth month with respect to the current month. Below, [-1] shows the immediately previous month. These offsets are limited to 60 months in both directions.
The use of the [-1] means I am referencing 1 cell to the left. You can also drive your formula on an average of months, in this case you will need to use 3 offsets: [-1], [-2] & [-3} then divide by 3, the total months.
Once you have entered your formula in correctly, you can copy the format over and it will apply the offset to future months.