Skimming is mentioned in the LA535 instruction book, but probably not explained as well as I would like it to be. It's a way of taking thin layers of material off the surface as opposed to digging deep. I mainly use it for back-drag as well, but trying to enhance my skill set with it so it looks like I actually know what I'm doing. I added my Piranha bar just for the purpose of being able to move the hard soil in the summer and cut smaller roots when doing so.I only use float for back dragging/smoothing things out. What specific things are you wanting to use the float function for? I'd like to learn.
If you put the boom in Float, you should be able to angle the bucket down slightly to skim the surface, letting the angle of the bucket determine how much it digs in. By varying the angle of the bucket as you move, you should be able to make a fairly smooth cut since the tractor is rolling behind what the bucket edge is doing. However, since the bucket curl is just as sensitive as everyone seems to agree the boom is, I find it hard to do. But, the more I use the tractor, the less likely I am to throw myself off if I forget the seat belt. The first few times, I bounced myself (and the tractor) around quite a bit learning how to handle that joystick.
What I've learned about skimming, though, is that if you hit a small root or anything that might offer some resistance, the front wheels will come off the ground as you're pushing which A) makes the driver insignificant unless you're sharp with the cutting brakes, and B) changes the angle of the bucket making it dig harder. The harder you push, the higher the wheels lift until the cylinders stroke to zero at the head end. As soon as you back up, though, the front end will drop back down pretty quickly. I'm sure the skimming technique is better in loose material, but I've been trying to learn this as a means to take up top soil and spoil it somewhere so I can reclaim the topsoil onto any excavation I make to help the grass will grow back a lot quicker. And, like I said, make folks think I know what I'm doing.
OMH, the bucket level indicator, (the bent rod on the RH side of the boom), should be adjusted so that the bucket is level (flat) when you're sitting on a flat surface. It doesn't actually do anything for you, but it gives you a quick visual of the angle of the bucket. The bend in the rod should be centered in the guide it runs in. If the bend is on the far side (bucket side) of the guide, the bucket is angled down, and will dig if the boom is down or spill out whatever is in the bucket if it's lifted. If the bend in the rod is on the operator side of the guide, the bucket will slide on the surface and hold whatever material you have in it (assuming the bucket is not overfilled for the current angle) when you lift the boom. It's just a quick visual without having to try to see the bucket, and even if you can see the bucket, it's often hard to tell how close the cutting edge is to the dirt.