After playing your game, if I understand you correctly. All you want is a crosshair to be drawn where you would theoretically fire your bullet correct?

If so, it has nothing to do with Pi or Radius. These are the equation you wanna use R*sin(theta)=Y, R*cos(theta)=X. Where R would be the distance you want the crosshair to be from the turret if the turret is either pointing directly up (90 degrees) or directly to the right (0 degrees). Theta is the angle which the turret is pointing at (The one you're currently displaying in your game.) Y and X are the respective cardinal coordinates of the cross hair.

Example: R is 20. Theta is 35.

Using that information R*sin(theta)=Y -> 20*sin(35)=11

R*cos(theta)=X -> 20*cos(35)=16

To check for accuracy we use the Pythagorean theorem 11^2 + 16^2 = 19.41. Considering I rounded the first time, it's close enough to 20.