Does the rifle have a round receiver? I'd say measure the diameter at the front receiver bridge and divide the measurement in half. Measure the height of the rail above the receiver. Then, either use the factory measurements for the rings or measure the distance between the top of the rail and the bottom of the scope tube. Finally, add half the diameter of the scope tube.

This assumes you have dial calipers. If not, get some! Even the cheap ones are quite useful.