-1 to -64 is the reverse speed...
Yes, of course, but a value of, lets say, -10 is just the same speed as +10, but in the opposite direction, no?
If you want to increase the reverse speed, you send it a more negative number, right?
It would be worth a shot to try it. Just make all the values you pass the same as you would pass if it was a forward speed. Negate the result and set the speed. See if it works. It's what I would try right away, partly because it makes sense, and partly because it's dead easy to try.