Different apparent wind, yields different heel angles.
Your optimal heel angle is dependent many things, but the bottom line is the optimal heel angle is affected by strength and direction of the apparent wind.
In light breeze you want a a slight heel and often actually have to put weight to the leeward to cause the heel when heading up or down wind to keep the sails filled. In heavy air upwind the boat wants to naturally lean, but usually too much so that is why everyone is hiking out on the windward rail. Down wind in heavy air, your typically going near (or greater than) hull speed, so a flat boat is desired.
However, the most simplistic answer is that the optimal angle of heel is that at which you have a balanced or neutral helm. In other words if you let go of the helm, the boat would track short straight for a distance without rounding up or falling off.
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