well the fact that there is already a valueOpMinus operator involved complicates things a bit, I would need to know the whole rundown of what you want to do and how you want to time it in order to advise you.
but another thing you can do is create a dummy dial that is only there to change the forumula of a rotational or translational dial to one that is easier to work with by setting the dummy dial up with erc code that refrences rotation so that it can give a 0 to 1 (or whatever other number range you need) result and do your subtraction from that.