It often is said that the thinnest layer of shellac that forms a continuous film is the correct amount. When spraying something like Sealcoat three coats should be plenty enough to do that in most cases. But when I'm using it under a WB topcoat I typically spray only one coat of shellac, then proceed with the WB. The only time I spray multiple layers of shellac under a WB topcoat is if I'm using it with Transtint dye as a toner where I might need to spray two or three coats to get the color I want. I recommend you keep it simple. Spray one coat of shellac and then move on to your WB topcoat. I think you should be able to cover around 400 FT^2 with a gallon of Sealcoat.

Neither shellac nor (most) WB topcoats are particularly dangerous to use, but do wear a respirator with organics cartridges when spraying them and provide good ventilation. I've read the hangover from inhaling alcohol fumes is really bad.

John