The boat will tell you
As you gain experience, the boat will tell you. As you develop a feel for your boat, you'll notice that as it heels too much, it will be harder to steer and will develop what is known as weather helm, which is a strong tendency to want to head up into the wind. You'll learn to notice that there's a point where everything feels right, the wake is gurgling, you can feel the power in the sails and the boat is surging forward. You'll know then that you're "in the groove". The advice to sail as flat as possible is good advice in stronger winds. In lighter winds, a little heel will make you faster. You can do this by shifting crew weight to the low side. In extremely light winds, adding some heel can put some shape into your sails, giving the light wind a better airfoil over which to flow. Think of sailing as a bit like riding a bicycle. Once you have the basics down, the rest is by feel. When you're cornering a bike, you don't think about what the proper angle is for leaning into a curve, you find it by feel. If you find yourself wondering if you should reef your main, you probably should. If you're wondering before you leave the dock, do it there. It's much easier to "shake out" the reef later if you're underpowered than it is to put one in under sail. Practice reefing until it's second nature. When you need it, conditions, by definition, will be a little rough. One final, and most important thought. Even though you might think that you're not heeling too much, if someone else on the boat does, slack off a bit or reef. It's not worth making a crewmember, guest or spouse uncomfortable.
