Most modern genoas have three pairs of luff telltales, an upper middle and lower telltale. In most conditions you want to place the sheet leads (blocks) so that all three telltales break evenly, meaning that the top, middle and bottom of the sail luff at the same time. If the top telltale is luffing and the bottom is flying properly you need to move your lead (the block position) forward. If the lower teletale is luffing when the upper is flying then you need to move the lead aft.
There are exceptions to this. In heavy air in particular you may want to move the lead slightly aft of its apparent optimal position so that the head
of the sail is twisted open which helps reduce heeling. In light air the sheet lead sometimes needs to move aft as well to compensate for gradiant effect.