A good number of guys approach the problem of applying the right amount of resin from a slightly different angle.

They apply the resin to the glass on the model using any method that seems right. Then to insure there isn't too much resin they roll the surface with a roll of toilet paper. Put down a layer of paper. Soon as it's down, peel it off and it takes some excess resin with it. Trash that and roll it again. When it looks uniform and looks like no resin is puddled anywhere, you're done. You got a lot of resined toilet paper in a pile, but no excess on the plane.

Like almost everything in our hobby, you gotta get the feel for the job. But it works once you got the hang of it.