slack in the forestay is good. that means your jib wire is as straight as possible . now you want to look at the distance between the blocks of the main sheet when you are sailing hard to windward. the space between the upper block and the lower block should be minimal, with lots of tension on the main sheet and sail. if you are "block to block" before the jib wire is fully tensioned then your jib wire will slacken and your jib will become too full. the forestay will straighten and the jib wire will curve. thats slow upwind. if you fully tension the main sheet and you have space- more than 1"- then you dont have maximum mast rake, which is also slower upwind. optimum is some slack in the forestay, jib wire carrying all the load (straight as possible) and main sheet blocks almost kissing. most racers use a 6:1 mainsheet system that allows more rake than the older 5:1 stacked system. am i making sense?