Properly adjusting the fore and aft position of the jib blocks ensures that your jib draft is steady from the top to the bottom which ensures that your jib is at full power. The best position can change depending on wind strength and the shape/condition of the jib. In order to determine the best fore/aft position of these blocks you should have both an upper and lower set of tell tales properly installed on the jib. While under sail, gradually round your boat down onto a deep reach and watch to see when the outer tell tales stall in relation to each other. If the upper is stalling before the lower, it means the top of the sail is flatter than the bottom and you need to move your jib blocks back slightly in order to get more sheeting pressure toward the bottom of the sail. If the bottom stalls before the upper, it means that the bottom of the sail is flatter in relation to the top and you need to move the blocks more forward in order to get more sheeting pressure toward the top of the sail. You can also check your adjustment by luffing the boat up into the wind to ensure the inside tell tales also react in the same even manner.

On boats with limited fore and aft adjustment of the jib blocks (like ones with the blocks attached to the front beam), it's possible to make the same kind of sheeting angle adjustment by raising or lowering the position of the jib on the forestay.