If you have a set of calipers which measure inside/outside measure the web between the chambers just ahead of the rim seat & then measure the outsides of the chambers, also just ahead of the rim seat. add the two figures together & divide by two, this will give you the spacing of the centerlines of the chambers, firing pin spacing will be very near to the same.
While you are measuring measure the diameter of each chamber, again just ahead of the rim seat. If its a 12 it will be around .812", if a 10 around .860".
