So we'll say you have an accumulated 170 gallons of water. If you are going reef and will eventually have an adaquate amount of live rock (1.5 to 2 lbs per pound) and since tangs aren't the dirtiest fish, infact they are an advantage... Taking your other fish into consideration also, I would, and keep in mind that I said "I would" attempt around 28 inches of fish total. That's measured by the length they will get, not what they are now...
You now have a regal so that's 12 inches.
a yellow would be 7, a powder blue 9, a sailfin 15 and a naso (which I think are so cool) will get to 18 inches.
Don't forget to add your other fish into this equation.
Oh, this is about 6 inches per gallon, because that's the concern here... What your bioload will be able to handle.
If you want to attmept 4 inches per gallon then you'd get a few more inches, but I would (<---pointing out again, this is only my opinion
) not attempt any more than that, just to stay safe.