Just setting the rear sag and it's far more than it should be. Would anyone know the length of thread of showing above the lock ring at full pre-load?. Appreciate its not a good idea to fully pre-load the spring but it will need a fair few turns to improve things and can't see where I am at because of the top shroud/cover.

If you have too much rider sag then the spring rate is incorrect. Winding in preload will only increase the ride height and continued increase in preload will eventually top out the shock when the suspension is unloaded, which is an undesirable outcome. How much rider sag do you have?

50mm at the rear and its road riding only. I don't want to apply too much pre-load to reduce this to 35mm ish for the reasons you have given but would likely be happy using 2/3rd of available adjustment which should improve sag figure, but can't see where I am at due to cover on shock. The previous owner had it set with about 10mm of thread showing and that gave 50mm sag with me on bike. Might need to wind it right in and see what the adjustment range is and then come back from there to a compromise sag figure without too much pre-load, it might work out?

I am now on a diet

An increase in preload will increase the loaded ride height, 5mm may give a 15mm increase depending on the suspension ratio but the soft spring will still give a lot of squat under acceleration and possibly top out under deceleration. Neither of these states is desirable.

Increasing the shock pre-load did help a little, but bitten the bullet and ordered a new shock and the fork spring upgrade.