I'm assuming you typed N+1 in error, when you meant N-1. You certainly would not want to dilute a developer for N+ development.(posted 8287 days ago)But assuming N- is what you want, diluting your developer is the best way to go. I've found much more pleasing results from going this route, than by simply reducing development times. The later typically does not separate the midrange values well and leaves a rather muddy, dull looking result. Dilute developers compensate, holding down the highlight development, while permitting the midrange and shadow areas to develop more fully.
If you want info on using TMAX films, check out this site: http://www.cs.berkeley.edu/~qtluong/photography/lf/articles/sexton- tmax.html It offers a wealth of useful information of processing TMAX films and N+ and N- development times. Incidentally, John Sexton, who provided the information is Ansel Adams former assistant, and a masterful technician in his own right, so he knows of what he speaks.
Of the Kodak developers, TMAX (roll film ONLY) and TMAX RS (for sheet film or roll film) are favored over HC-110 or D-76. If you are going to the trouble to work out development times, you're probably better off going with TMAX developer instead of HC-110.
If you want a general rule of thumb (and understand I have not tested this specifically with TMAX or HC-110) cut the concentration to 1/2 (example: use 1:15 instead of 1:7 from working stock, not concentrate, of HC-110), reduce agitation to 10-15 sec/3 min, then INCREASE development time by 60%. That will put you in the ballpark for N-1.