From Godfrey Oxley-Sidey’s archives

Personally  I now use Michelin XCA, (a  rather heavier type of X tyre which was a long way from being invented in the 1950s,) on my  400 with benefit though I usually run them a little higher than the 24/27 suggested  below for the 600/16 X back in the day. Empirically,  I tend to use 28psi  all round for road use, up to 30 all round for a track day.