XxHeavyRainxX

The obvious only option for crouching in PC games is Left-Alt. ;)

Reasoning: The only thing your left thumb is doing is hitting the space button for jumping. But crouching and jumping never happen together (outside of maybe Half Life). So the thumb is for crouching and jumping!

Pinky: Sprinting (Shift)

Ring Finger: Strafe Left (S)

Middle Finger: Forward & Backwards (W & S)

Pointer Finger: Strafe Right (D)

Thumb: Crouch & Jump (Left-Alt & Space)