This is to show the difference in the final shape of the sleeve-head if your points G and J are significantly different.
This is my sleeve-head, and you can see that the pink line (Cap Height J, the body measurement) is significantly longer than the black line (Cap Height G, the ideal measurement that will result in minimal ease in the head of the sleeve).
The length of the sleeve cap for Cap Height G is 19.7 inches, and this include the ease required to sew the sleeve into the armhole.
The length of the sleeve cap for Cap Height J, there is an additional 0.89-inch (2.25-cm) ease. This will result in gathers or tucks, unless a dart is put in or a seamline (resulting in a two-piece sleeve).