Hi
Mines 17.7mm (actually 17.69mm), compared to an S4 back to back guage given to me by Ray Hammond which is 17.84mm.
Hope that helps
PS: The leaflet in the box of the P4 gauge states BB max: 17.75 17.67 min
If you obtain the GW Models back-to-back gauge, you can set it to whatever you want, within limits. I seem to remember that there was recently a discussion in S4 News about the best b-to-b measurement, so might be worth a look.