Hi Ian (R) - welcome.
I used to run my '53 NH at nil and 2 thou - as I remember in practice that resolved to just 'disappearing' the clearance but still able to slide the rocker across the end of the valve cap for 'nil', and just able to feel the slightest bit of clearance for 2 thou (and making sure that the exhaust lifter was well out of the way) .
Ran that engine for a year or more as the 'daily driver' - including some very long excursions, never had any trouble with the valves or timing gear.
However, always start off with appreciable clearances if you've renewed the rocker-box gaskets as they will sink pretty quickly initially (unless maybe you're using Drags' ally gaskets).
The pre-'53 iron engines (or is that pre-52??) were nil and nil, I did a lot of miles on my '51 VH using the nil/nil settings, again no problem.
In my experience, if you've set the tappets slightly tight (or they've got tight because of the rocker box gaskets compressing) you find that you've got good compression when hot but poor (or very little) compression when cold, and I'd say you can use that as a yardstick for whether you need to slacken something off slightly. (I guess that also reinforces the theory that the clearances increase as the engine warms gets hot!)
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)