Generally, I'd say with most developers about 5 minutes at 20 degrees seems adequate.(posted 8106 days ago)Try developing as per TMax 400. I've seen it done the other way around: TMax in ID11 etc, developed for the same amount of time that HP5'd be developed for.
Just one question: why not develop in TMax developer? TMax was designed as a silver iodide film and really needs a silver iodide developer. What you're doing is developing a silver iodide film in a silver halide developer and thus throwing away the advantages that iodide gives with its close T-grain structure.
Rhys