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.
Bristol Owners Heritage Trust 