A heavy attack that is not a fully charged heavy attack is often referred to as a medium attack. The longer you hold the mouse button down for the more damage the medium attack will do but also the more dps time you "waste" charging it. Light attack weaving offers the most dps but is also the trickiest to do consistently; it takes better timing to light attack weave without messing up than to medium weave without messing up, at least with a fire staff.

If I medium weave I will hold the mouse button down for just the right amount of time to trigger a medium attack (2 fire balls), which is just fractionally longer than a light attack. I think this does slightly less dps than light weaving but there's not much in it, and your dps while light weaving depends on how consistent you are. So you can decide to light weave and possibly mess up more depending on your dexterity, or medium weave and be more consistent. I tend to medium weave when solo, like for vma, and light weave if I'm in a group and have less to worry about or need to move around less. But if I'm mostly medium weaving I might occasionally throw in a light attack if I want to get the next skill off as quickly as possible. Practice both to learn the timings of light and medium attacks so that you can just switch between them as you see fit.