correct Chris, I didnt realise you want the toroidal on a vertical mount the one I have for horizontal mount is a centre filled 4 hole at 650 dia by 270 high and quoted 77L capacity (61L usable) I would imagine the useable figure would change for an upright mount,not knowing the reason we can't make use of the full fill capacity of the tanks at the working pressures involved I wouldn't go beyond the safety margin if that is what is
That's exactly what it is. It's to ensure the tank always has some "spare" volume containing only vapour (as opposed to liquid).
If the tank were to be filled when cold (to 80%, say) then heated (in the sun during the day, for example), the liquid would expand to take up a greater volume within the tank.
Provided there is some volume containing
only vapour, all that will happen is that the pressure will rise slightly, and a little of the vapour will condense to liquid, reducing its' volume, making space for the larger volume of liquid without greatly increasing the pressure in the tank.
This balance of liquid and vapour in the tank keeps the pressure more or less constant (but still temperature dependant) regardless of the volume of liquid. It's a bit like the boiler of a steam engine. You have a volume of liquid that's
well above its' boiling point at atmospheric pressure, kept liquid by the pressure of vapour with which it shares its' surroundings.
If you fill a tank to more than 80% you are left with a smaller "buffer" of vapour so the tank can cope with less heating before you use up all the available volume.
Once the vapour volume gets too small the increasing volume of liquid cannot be accommodated so the pressure will rise much more markedly if the tank gets hot, and it may cause the pressure relief valve to vent.
80% is probably a pretty generous margin for our climate, where the temperature differences aren't great. I believe Calor gas cylinders are filled to 87%.
The problem is, the filling cutoff is something that
should be tested during certification of an install, so tampering with the valve to get a higher fill level might give you a headache. Then again, unless the guy certifying the install does the "first fill" he's unlikely to notice, I suppose.

Kevin