I've seen similar question before and I think the answer is you can't because it's a webgl thing, thus it is not accessible through system expression. I can't find the thread but if I'm not mistaken, there's a workaround by assigning the effect parameter with instance variable or normal variable. This way, you can get the effect value through variable.
I'm probably wrong on the webgl thing, probably Ashley himself can explain why it is not accessible.
Btw, it is interesting on how do you intend to use it in mathematical expression, can you share what are you trying to achieve?