How do you know when to pull your sail in "block to block" (when your mainsheet is in tight, and the traveler blocks are touching)? I don't think there is a rule of thumb for this in any condition. More so, in tricky light wind conditions, I think it's even less predictable. I would recommend you to take your attention off of when and turn it to why.
The Laser sail has a very closed leech, which means that the curve from luff to leech is similar from top to bottom. This means that the wind has a lot of distance to travel in order to keep its attachment and proper flow from luff to leech on the windward and leeward side of the sail. In order to trim the sail to your needs, we must consider the speed at which the wind is flowing over your sail and the angle at which it is hitting it.
If you aren't already familiar with apparent wind directions, it is the combined angle of the true wind direction and the artificial wind created by the forward movement of your boat. As in any other boat, Lasers sail by the apparent wind angle, rather than true wind. Because of the curvature of the sail and range of speed in different conditions, it is very easy to become stalled. Stalled is when you lose flow around the leeward side of your sail. To recover flow, you can either head up, sheet out or both.
A velocity lift is a sudden increase in wind speed (puff) that temporarily shifts the apparent winds away from the bow.
Oftentimes, elements are changing the direction of the apparent wind without your notice. As your boat speed increases, your apparent wind is shifting forward, and you can start bringing your sail in to match the angle of entry. If some chop hits your bow, your boat speed slows (less boat wind) and your apparent wind moves aft toward the dominant true wind direction. Your angle of entry moves back and wind flow over the back of your sail decreases, sometimes lifting your leeward telltale, but not always in most cases. Often confused with pinching, which is exactly the opposite (an increase of flow over the leeward side of the sail commonly noticed with backwinding), or the windward telltale flipping up. In the case of stalling, speed up in light air. In the case of pinching, BEAR AWAY.
We are constantly changing gears to keep the boat balanced and quick. every time you think a wave, or set of waves are slowing your boat down, even by the slightest amount, make an adjustment by sheeting out with your mainsheet to maintain flow over the leeward of your sail.
If you bear away with a stalled sail, you are becoming increasingly stalled, which oftentimes results in loss of pressure, windward heel, and leeway. Very common is a velocity header or a velocity lift. The velocity header is a sudden drop in wind speed (lull) that temporarily shifts the apparent wind moves forward, towards the bow. This will give the appearance that you are pinching. Remain calm, as your boat speed slows down, apparent wind moves aft again and there should be little change to your balance. Release the cunningham to open up your angle of entry, and maintain speed and pointing.
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A velocity lift is a sudden increase in wind speed (puff) that temporarily shifts the apparent winds away from the bow. This usually gives the appearance that the wind has shifted aft and that you can head up. Your best course of action is to sheet out and get your boat up to speed as soon as possible, bring your apparent wind forward, then sheet back in to stay flat.
In summary, we are constantly changing gears to keep the boat balanced and quick. every time you think a wave or set of waves are slowing your boat down, even by the slightest amount, make an adjustment by sheeting out with your mainsheet to maintain flow over the leeward of your sail. As you speed up in light air increases, begin to trim your sail in to match the apparent wind moving forward. And every time there is an increase in wind velocity, sheet out to match your boat speed, instead of heading up right away.
