I haven't been using T-max developer any more lately, but the dilution sounds right, and the time does seem plausible, too. Can you exclude other sources of error, such as developer exhaustion?(posted 9167 days ago)I remember that 1 l of t-max 1+4 is supposed to last for a dozen 35 mm films, but you have to develop the 5th to 8th films one minute longer than the first four, and the 9th to 12th another minute longer!
Also, T-max conc. in an open bottle won't live forever!