Yes, by feel. When I first started that was one of my worst fears, I barely knew what I was doing ,not making enuff slack in the pocket area, doing the middle pockets, the trickiest ones , I'd get it just about right and then #@%! damn I just put the staple thru the cloth.
Just let the gun rest on the rail or framed slate,do not apply any pressure,or very little,make sure its flat or it will kick one side of the staple out, It takes a while to get the feel of this gun but besides a few jam ups now and then it's a very reliable gun ,give it 2 small squirts of wd40 before every other use, one in the magazine area and one where the driver blade/plunger comes down.
Use the longer staples in the pockets(unless/until you can do them easily) and the 1/4" ones for the rails and framed slate. I have two of these guns ,one has about 900 tables on it and the new one has about 175 ,I keep the old one at my shop when I do rails there and I bring the new one with me.