The centre wheels don't make contact as you know and are there for cosmetic reasons only. I normally set my back to backs with a clock vernier at 14.6 mm
sometimes a little more, I've even read of some people using 14.9 mm. I find this more reliable than using a back to back gauge. You might find your example is a little tight on back to back. The wheels are quite easy to push in or gently ease
outwards with a screw driver. Of course without a vernier your guesssing as to what the measurement is exactly.
