It's good to know whether your gas consumption rate is higher than the assumptions made in the numbers Rainer has given you, because for some very new divers, an UNSTRESSED SAC rate can be 1.0. But otherwise, knowing your precise SAC rate is unnecessary for the "rock bottom" numbers. It's really used for planning.
And "rock bottom" applies to ALL dives, not just boat dives. If you swim out from shore to 80 feet of depth and your buddy's reg freeflows and you need to share gas with him, you STILL need enough gas to get him AND you to the surface, don't you?
You are troubled by the fact that rock bottom for deep dives is a large proportion of the gas you take with you. That IS troubling, and it should be. What it tells you is that, when you do deep dives on small tanks, you either get a very short dive, or you play Russian Roulette with your gas. But do remember that rock bottom is a dynamic number during the dive -- if you spend a little time at 100 feet, but then you move up to 80, your rock bottom has gone down from 40 to 30 (what that means is that you have YOUR part of the original rock bottom now to use). If you then move up to 60, it goes down again. So, at each depth, you have a number where you have to begin to ascend -- but if you get to the next depth without having to share gas with anyone (as you usually would) you have some more gas to play with at the new depth.
So I guess what I'm saying is that, if you plan a 100 foot dive on an Al80 (which I wouldn't, but it's up to you), you don't have to get back to shore or back on the boat with 1600 psi. You don't even have to do a direct ascent from 100 feet if you hit 1600 psi. You need to move up to where you have more gas than the rock bottom for that depth, and then keep doing that, so that you always keep at least the reserve you need for that depth in your tank.
"Turn pressure" is relevant to all dives. If you are doing a drift dive off a boat, you have all available gas (everything but your rock bottom) to use. But if you are doing a dive off an ANCHORED boat, you at the very least want to divide your usable gas in half, so you swim away from the boat with half of it, and come back to the boat with the other half. If you really, truly HAVE to make it back to the boat (open ocean wreck dive, well offshore, anchored boat) then you need to think about being even more conservative, because you might have to get you and your buddy back to the anchor line, as well as ascend along it.
The numbers you're staring at are the reason why a lot of people go to bigger tanks.