With the Zone System or BTZS you do not stick to box speed. Why? In the days or yore when film was king, most film was bought by amatures, newspapers, commercial photographers. The manufacturers, although they used ASA later ISO standards they tended to error on the side of film speed rather than shadow details. Fine art photographers like AA and Minor White were less intrested in film speed, AA hated dark shadows, he wanted shadows with texture. Tmax 3200 is a case in point. Most who have tested Tmax 3200 find it have a true speed of 1200 to 1600, but Kodak DX codes it as 3200, it is intended for action not fine arts. I was bit surprised that when Kodak Alaris brought TMax 3200 back to the market that they did not market as 1600, with Delta 3200 still on the market I guess they did want seem to seem as slower than Delta. Finding the correct E.I is needed to maintain open shadows, then use a spot meter or get close enough with a reflective meter for your visualized shadows, usually zone II or III, then meter for your visualized highlights, record and develop to matched the visualized highlight.
BTZY does away with most of this thinking. Phil Davis does seem to care much for visualization, he wants a negative that provides all the possible tones for Z III to Z IV the zones with texture. His system uses an indecent meters, he meters the shadows and highlight to determine the Scene Brightness Range, that data along with other data goes in to Android App that spits out the exposure and development time. In BTZS you do test for your film and developer combo, needed for the App. I do not use BTZS, but I have seen great work. The View Camera Store is an online shop that sells all things BTZS including testing fo sheet and roll film.
There are many people who take part of the Zone System and call it the Zone, it is not the true system. It may work for them, depending on preferences. As a former PJ when shooting 35mm I don't pay much attention to shadow, I'm looking for action, a moment in time. 35mm landscapes, I shoot close to box speed and use matrix or incident metering. With sheet film, something times I shoot with an incident meter, using my established E.I. When I'm in a zone frame of mind, back the basics and I follow Minor Whites approach.
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In my testing my 4X5 lens with Tmax 400 and DDX, based on a shoot around, I do get an E.I of 400. With Foma 400 200, 100 60. Ultrafine and Kentmere 400 work for at 320 in MCM 100 and D76, at 600 in Diafine and Acufine. Another person may or may get the same results. Leaf shutters do age, finding repair techs to keep old shutters in tolerance gets harder and harder. And because I shoot a ring round who knows what my aging eyes actually see.