For the tmax you can use a three position switch and two sliders. The three position switch does the standard running tanks in and out, one slider proportionally controls the last 20% of the tanks and the other slider controls fore/aft trim
So you flick the three position to go down, then use your slider to achieve the perfect bouyancy sliding it down. Then the other should be centred unless you need to shift the trim. You can tell how far the tanks are run out from how far the piston shafts are sticking out