How do I apply pi to work out a radius measurement on a drawing?

It is unlikely that you will have any measurement on the drawing that is related to a radius by pi. You double the radius to get the diameter, and multiply that by pi to get the circumference, but you are very unlikely to get a drawing in which the circumference is given, but not the radius. The radius is the gap you would set on your compasses to draw the circle. If you did need to design a wheel of a given circumference, maybe for a gearbox, then you would divide by 2pi to get the radius needed.