Add drop-downs with absolute numbers to "plus"/"minus" buttons.
My initial brainstorm: Rather than doing multiple clicks, display a drop-down with absolute numeric options such as 1,2,3,4,5,6,7. For example if 4 generations are currently revealed, clicking 2 would immediately reveal only 2 generations. Any single click on a button would change the reveal by a single unit as is currently the case.
Alternatively, combine the buttons into a single +/- button. The drop-down could have two vertical columns of numbers. The leftmost would be positive numbers and the rightmost would be negative numbers. That would allow the user to perform a single click on the button and draw down to the desired number and release the button to execute.
I meant to say "drag down" in last line.