With a table holding a benchmark between two filters like shown below, there are three smart calculations that will fit the scenario.
You can add Difference, Growth percent, or Growth index to this (depending on what suits your purpose) - and it will come out with a calculation that shows the trend for each item:
The prerequisite is that you want to show the trend from 1st to 2nd column.
If your scenario is the reverse (in the example above with year to date first, you will need to open the calculation you have added, eg the difference like show here:
d-1 actually means the 1st column from the right, and d-2 means 2nd from the right - so if you swap those two references your calculation will fit that scenario.
So - even without a perfect understanding of the TARGIT syntax, you will be able to calculate benchmarks, supported by smart calculations.