I was the poster in question.
Perhaps "know" was too strong, and "good estimate" a better description? Obviously I don't have a pressure transducer connected from the tank to my head so I don't
"know" the pressure in the tank.
I can, however, make a good estimate based off of knowing my normal air consumption and then factoring in what's happening on the dive.
As to fairybasslet's comment about instabuddies--the simple answer is to avoid them and dive with regular dive buddies. However, if you're going to dive with someone new, ask them what their air consumption is. If they give you a number, at least you know they're halfway thinking. If they look at you with a question mark on their face...:11:
If an instabuddy gives you a number, when you check your own gauge during the dive run a quick calculation based on the number they gave you and the current dive conditions, and ask them what their pressure is. If it's off by a good amount, either they were lying or perhaps they're not as comfortable as normal. Either way if I'm diving with someone new, I'd rather dive with an instabuddy who gives me a number rather than one who doesn't.
As I said in the original thread, I have less than 100 dives. I have a
long way to go in knowing my air consumption in different conditions (that's why I'm OCD about keeping track of it in my dive log). I'd say I'm within +/- 100psi at least 80% of the time. Other times I don't get it quite right.
For the diving I'm doing right now, that's fine. Later on that accuracy will increase with experience and knowledge.