I've never had a problem with conflicting, or unreliable info from Drs. Foster and Smith's website, www.liveaquaria.com
So far everything I've looked up there has been spot-on with the prevailing wisdom on all of the aquarist forums I frequent.
Using your example of a Sailfin Tang, their website says it will grow to 1' 3" and needs a minimum of a 125gal tank.
That matches up perfectly with just about everything I've read and what the veterans here are saying.
It only makes sense considering Tangs in general are extremely active swimmers, and this one in particular gets to be quite large... and remember, those recommendations are for the bare minimum
that's acceptable to humanely keep that species.
Of course, larger would be even better.