ah but there's the thing about acceleration, Bud. it accelerates. it's increasing by 1 inch every three years right now, but if the rate continues to increase over the next 100 years, as you describe yourself, the net change can be much greater than three feet. in other words, acceleration curves slope upward exponentially.
it's hard to be exact with this kind of non-linear effect, but the net increase should be more than three feet. maybe not twenty, but maybe even a bit more. i tried to make a conservative estimate, but i know it will be off at least a little.