[Graham]

Logically speaking, you are of course correct so I have accepted that I made an error of judgement and amended the Scrutineer report above for Class B cars, and we will use the conventional understanding for Tyre widths. So penalties may apply at the end of the series to the non compliant cars in the class (including my own)

Its fine with me as was, you are running the show and so your rules are what we adhere too, it was just my opinion not a protestation.
Given the time and care to organise these things, I am only too happy that you are doing this, as I thoroughly enjoy reading all the updates and preliminary reports 

As an aside, how do you determine the ground clearance ? I purchased some small squares of 1, 1.5 and 2mm sheet metal to check on a set-up plate, and then discovered my  metal thickness was not as advertised, so in the end it was a bit of guess-imation 

The problem with rules is that you can often find different ways to interpret them, but as I said I think that normally when I been entering National Competitions, it would be the "wall thickness" which would be the law, I have heard before folks protesting to the scrutineers using the argument about track contact area, but the organisers remaining unmoved by the protestations   So I probably should know better.

But more importantly especially with the classic Class cars the tyre width performance is probably a lot more critical due to car stability than with the other Classes where other characteristics of performance like overall car width or motor choice might be the dominating performance effect.

I am using a Scaleauto gauge to measure track clearance, it is not perfect if I check it with my digital caliper, as I have found that the increments on it can be plus or minus 0.02 mm out, depending on which step you are using.