You probably just need to clean or calibrate your verniers as standard BMX (and Campagnolo) crown race ID is 26.4 and non-BMX is 26.5 or 27.0.

Why not just use the crown race from the BMX headset? You could use the punch method (it does work quite well) or red locktite. Also, you could try making a shim from the appropriate thickness feeler of a cheaper feeler gauge set. Or blow the holiday drinking budget and buy a Campagnolo crown race.