Calculate when to decelerate ship rotation to stop at a certain rotation

[Xty]

Can anyone help me out with the math and stuff to figure out when to decelerate rotation to stop at a certain position?

I've got these formulas:

a(t) = dv/dt

v(t) = dx/dt

I think I can figure out the angular acceleration, it would equal my ships turning torque / its moment of inertia I believe so I have acceleration. I know the point I would like to stop rotation at and I know the end velocity would be zero. What I don't know is what time I would start decelerating or what position.

[notfirestorm]

Depending how crazy and complicated you want to get, you could go all the way into Control Theory (http://en.wikipedia.org/wiki/Control_theory) and utilize feedback loops, PID Controllers, error signals, critically damped systems, and so on. The math can get crazy, and I forgot most of it years ago.

Or, depending what you're attempting, you could go much simpler. If you're trying to do an autopilot type thing in space, when there are no other forces acting on your ship, you could just figure out where you're pointing, where you want to point, and the path you will trace out trying to get to where you want to point. Then, using that traced path, find two points on that path equidistant from the halfway point (like the 25% way there and 75% way there points). Then you can just torque towards the end position until you past the 25% point, stop torquing until you hit the 75% point, then start torquing in the reverse direction until you hit your target position. If you're not doing anything else while doing this rotation (like pumping, burning, or jettisoning fuel) this works surprisingly well for how simple it is, since you'll spend the same amount of time accelerating and decelerating your angular momentum

[Xty]

What I was hoping to do was figure out the angular acceleration a ship can achieve and use that know when to decelerate the ships rotation. I have already used the cross product of the direction the ship is facing and the target direction to figure out which way to turn.