[morzh]

Since you are dropping almost 85% of the voltage you can roughly say that decreasing Rc by x% will raise the load voltage by the same x%.
So if you wanted your fil. voltage to be 18 V it now will be roughly 20V.

But since you drop most volts across the cap, you can add a little resistor and correct it.



As for the math, it is really arithmetics here, the most complex operation being the square root. 6th grade if even that. I am sure this can be understod by anyone once the fear of using calculator is overcome.