I've never run any strictly controlled tests, many years ago I simply noted exposure times indicated by my meter and made a exposure then carried on doubling it, exposing about 15 or so sheets of film in the process. I learned a lot from that excercise, doubling the exposure each time is only one stop increase which is not a lot when considering bellows extention etc when using 4 x 5. When I developed them I selected the negative that looked right, good detail in the shadows and noted how much more exposure I had given compared to the first meter reading. Thereafter that was my starting point when making long exposures. For quite a long time I did bracket but as I gained experience I began to understand the characteristics of the films I used and I never bracket now. I always make two exposures but they are the same and are made simply as a cover in case I happen to damage a negative when processing.

I see you use Tmax, I have a reciprocitry table that John Sexton uses so I'm sure that it will be good, although I've never used it for I don't like Tmax film. I'm not at home at the minute, I'm in London doing a one on one workshop in London and my client has gone to bed, but when I do get home I'll send you the table.