Okay, if he thought he was close to the deployment altitude then of course it's perfectly sensible to use the parachute. It's only if he was nowhere near it that I would question using the chute right away in response to vision impairment.
And I haven't watched any movies with skydiving, I just figured that if you were watching an altimeter drop you would be able to extrapolate for a limited time.
It's definitely an interesting question to consider. Our rule of thumb in skydiving is, "When in doubt, whip it out." You'd rather deploy too high than too low. There's a big hunk of granite coming up pretty fast that you don't want to interface with. :)
In Felix's case, deploying too high could be a huge problem as well. So I'd be very interested to hear why/how he chose to deploy where he did.
And yeah, under normal circumstances you could possibly extrapolate your altitude...but the problem is that once you can no longer see, you can't go back and get a good idea of where you were at prior to it happening. Visor fogging creeps up fast (even from 12,500'), and once it does, it's very difficult to even find a baseline from which to extrapolate.
And I haven't watched any movies with skydiving, I just figured that if you were watching an altimeter drop you would be able to extrapolate for a limited time.