I cannot stress the following enough:
If you're building a room from SCRATCH, do it RIGHT - hire an acoustician, one with experience designing studios.
You'll beat yourself up later on for not doing so.
DO NOT design a room based on criteria offered by members of an internet forum, whom you've never met in your life. I don't care if we're all platinum-album engineers here (I'm not ). There are experts on acoustic design out there, with years of study and experience. HIRE THEM - in the long run they'll save you a lot more money than you'll spend (on hiring them).

One last piece of advice: make sure the acoustician you hire has had previous experience building studios. You'd might want to take a trip to some successful big studio and ask for their consultant's name. I've heard a horror story once of an acoustician who designed mainly public gathering places (coffee shops, clubs, etc ... ) who made a horrible design for someone by choking his room with too much absorption and too little diffusion. So experience is important!