Just found a genuine pressure gauge as supplied in the original 11R tool kit.
http://www.ebay.co.uk/itm/Honda-CB900F2-Front-Forks-Air-Pressure-Gauge/200940214507?_trksid=p5197.m1998&_trkparms=aid%3D222002%26algo%3DSIC.FIT%26ao%3D1%26asc%3D17%26meid%3D8955063862114877729%26pid%3D100017%26prg%3D1006%26rk%3D4%26sd%3D200925430024%26#ht_34wt_1255