Can a calculated variable be used for controller input?

Let's say I have a target of removing 90% of the TSS, could I define a calculated variable as the removal percentage, and then use this variable as the input to a PI controller?

Thanks!

No. Calculator variables can be regarded as some form of post-processing. They are not part of the executable model and can hence not be used as input to one or the other block.

For what you want to do you will either have to create a custom block, or add top-level equations. The first should work in all versions of WEST, the latter requires WEST 2017 or higher.