How to input variables/values from existing equations into equation driven curve tool?

I am designing a spur gear and have created equations for the needed controls to make a configurable gear. I am at the stage of drawing the involute curve, but I want to input the radius of the start point from my global variables and equations, but the tool does not accept a variable for the radius. In my global variables I have R where R= "base circle@sketch1" / 2 When I try to use R in the equation driven curve tool as follows: x(t) = R * cos(t) + t * sin(t) where R is as mentioned above. I have the alternate formula in y(t), but when I use the variable I get an invalid equation error. \

Is it possible to make the tool take inputs from the equations?

SolidworksSketching