Your grip should allow your hand to lay flat on the ball (or as flat as physically possible), with the pitches of the holes for your fingers/thumb, allowing the ball to hang from your hand, without substantial grip pressure (NO Squeezing).If stiffness doesn't allow for "normal pitch", adjusting the thumb pitch, towards the fingers, as the finger pitches must be adjusted away from the thumb, should help control the ball with minimum grip pressure. Think of the ball letting go of YOU.
The pitch in your thumb can be increased, more angle toward the fingers, to improve control, as long as you can deliver the ball comfortably. If you are experiencing any discomfort try small adjustments (1/8) until you lessen or eliminate the discomfort and gain more control.
Lastly, adjust hole size so you feel slightly snug in the holes. The lateral pitches and hole sizes shouldn't bind the fingers. Adjusting angles of the holes and hole sizes are the last adjustments for your optimum fit. Don't be afraid to make small adjustments actively. As your hands change and are affected by temperature and humidity, often tweaking the "feel" is needed to maintain consistent execution.