Am I to understand that I can just use the function to save a value to memory without technically returning data?
That would be my take, using a global variable and sticking the answer into that. But I have no idea what this means:
we are asking it to return an equation not a value.
I think the only advantage of assigning the answer to a variable over a return, is that you can do as many calcs as you like and as many assignments to variables in a function, but it may only return one.
As to this:
set rpm of a servo
..... the purists will frown. A servo doesn't have rpm, it goes to a certain position. If the servo is modded so that it rotates, and the pulse you send it is now the speed, is it still a servo? At least one member here uses the phrase "continuous rotation ex-servo".
That raises another question: for one of those (ex-) servos, there is as far as I know, no specific correlation between the pulse width and the rotation speed. I suspect the pulse vs speed will depend on the mechanics of the servo, and may even vary across instances of the same model. So how do you know what pulse width to send it for a required rpm?