I don't normally post much these days, but thought I may be able to help on this topic.
The above posts are how many of us turn balls, myself included. I don't use a template. To turn a 4" ball I would turn between centres to 4" dia, mark a pencil line in the centre, then turn down each side of the line making sure that you leave the final shape in a parody of a rugby ball. ie- just slightly elongated 'over round' each side of the centre line. As above cut off the nibs that are left, fit your wood 'holders' and place the ball between them so that the pencil line is centred on the driving ends. ie- nib ends are running together on the centre line of rotation. 90deg to the first rotation.
Switch on and you should now be able to see a 'ghost' of the oversized round ends of the timber, all you have to do is remove this ghost carefully and you will be left with a perfect sphere.
Sand and finish! "Or as I sometimes do, re-centre another 90deg shift as I made an a*** of it, and you end up with a 3 7/8" ball.
Good luck.