I agree with Bob but in case they fail to do that, use a dial caliper or a micrometer and measure the thickness of the rod tip and multiply it by 64 to find the correct size of tip top needed. If it should come out (as it often does) to a number like 4.595, then you would be looking at using a size 5.0 tip top which is 5/64ths....(.078").

Most everyone that sells tip tops use the 64ths scale for them....