If you calibrate the 15mm ball, that calibration is only for the 15mm ball. I think it is possible to set up a probe as a reference in the Cimcore software, but I've never had to make a program with multiple tips for the Romer (one always covers it). Even if you do the reference, the correlation has to be established initially anways, and I would say it's very dangerous to trust the calibration of the 3mm tip by calibrating the 15mm instead.
I have noticed what you said about the 3mm probe not seating in the small cone on the bar because of the flat in the bottom of the cone. If you always apply force iin the same direction when you're calibrating it, you can minimize the error that's caused.
