It is generally feel. Pull more on as the wind increases. Different conditions require different approaches. But generally in medium wind...

If it feels like you are going slow, pull some on to flatten and reduce the drag of the sail.

If it feels like you are going sideways, pull some on to reduce power in the sail by flattening it. Also requires bearing off a little and moving forwards to increase boat speed and weather helm which will increase lift of the centreboards, rudders and hull.