Don’t forget the Hidden Hand of the Marketing Department. It must be very tempting to round up the figures, or, shall we say, be a little bit optimistic. Very few customers will be as picky and inquisitive as our more distinguished members here.
Paradoxically, the Zone System may collude in this. Remember, the System is intended to produce a personal EI for practical use in the field, and not measure an absolute film speed.
As an aside, I’m not quite sure that such a thing exists.
I think "Personal EI" exists in the sense that it can compensate for a variety of variables, among them the accuracy of thermometers, light meters, and timers, the behaviour of a particular emulsion to a particular developer, local water mineralisation, and so forth. Having done a fair bit of sensiometric testing, for all practical purposes - and assuming reasonably correct meters and such - I've found that 1/2 box ASA and underdeveloping 20% from recommended is a pretty reasonable starting point. After all, personal EI is just a starting point from which one departs for creative and interpretive purposes.
There are, however, two flies in this ointment. First of all - again after extensive testing - mechanical shutters vary considerably in accuracy, the higher speeds being the larger offenders. But even the lower speeds vary well away from nominal and the simple act of changing a lens can cause noticeable variation in exposure. Short of carrying a database of all my test data around, I just make sure
to (gasp) bracket somewhat around the indicated exposure.
Secondly, long subject brightness range environments are troublesome for Zone System. Although one can do N- development, to hold very long SBRs, this work at the expense of mid-tone compression (loss of local contrast) which is usually not desirable. This is what kicked off my 2021 year of study on the use of compensating developers with low agitation. Here you get your cake and can eat it to - holding a long SBR whilst actually expanding mid tone local contrast.
And, yes, no doubt the various marketing flacks and their minions were only too happy to declare that Tri-X had magically gone from ASA 200 to 400 overnight with no change in grain. It's always interesting to see the reaction of new photographers when I suggest the 1/2 box speed/20% under development regimen, when they first get back those negatives. For the first time for most of them, there is actual shadow detail.