Don't use the luff telltales when looking for the right position for the jib lead - the information you get is often more harmful than beneficial. The correct jib trim is always a combination of lead position and sheet tension, and there is no single right lead position. Come up a bit, or ease the jib sheet. In light air, steer just on the verge of collapsing the leeward tails. In medium air on an ideal heading the telltales jump up at three to four second intervals.
Telltales make trimming and steering easier.
One should not, however, stare his eyes out at them. If the cut of your jib is too flat, you will achieve the best speed by steering the boat so low that the leeward telltales are partly stalling. You should try to make the jib fuller by letting the head stay sag a bit. In strong winds, let the telltales live their own lives and steer the boat according to heel and sea conditions. When the wind sweeps past the sail its velocity near the sail surface decreases because of the friction.