The chart is in mm to help be universal because nothing else really is. Coins in Canada and in the US are different sizes in some cases and we could not even begin to make that a simple match to polyps sizes over seas.
One thing that does seem to be very universal in this hobby anywhere in the world is the mm measurement. Those of us in the US are the odd balls of the measurement world. We do inch, foot, yard, and mile, where everyone else does millimeter, centimeter, meter, and kilometer. But we still know what a millimeter is and it has the widest range of number in the small scale we are talking about here. We just need to come up with the areas in that range and put letters to them.
In truth, each area will most likely be a different genera and we would be giving a letter to that genera, but I honestly don't know that much about Zoanthids to know all the genera.