I've always felt that the upper end of a player's usable range is about a third or a fourth below the upper of their practice range.  With that in mind, I can hit an F or G in practice, but my usable range tops out at a high C or D.  It's not just a matter of being able to hit a high note, it's a matter of being able to do it well, at the exact moment when it's needed, which is significantly more difficult.

I've neve hit a DHC, even in practice.  Fortunately, there are many other things farther down the scale for me to improve upon.