I calculate as follows-- the text in the paragraph after this one is an extract from a document on estimating for furniture makers I created some time ago. It's a little on the generous side, but I find it's best to buy a little too much polish than too litle.
"The coverage of nitro-cellulose type finishes such as pre-catalysed lacquer and acid catalysed lacquer is approximately 60 ft² per litre, which is roughly the same as 6 metres² per litre. Applied as per most manufacturers recommendations of a wet film between 100 and 125 microns (µm) thick this gives a dry film thickness of about 50 µm after the solvents have evaporated and the finish is fully cured. Three coats applied in this manner results in a total dry film thickness of approximately 150 µm. Pre-catalysed lacquer type finishes should not be thicker than this because thicker films fail with cross grain cracking or the even worse ‘Chinese writing’ showing up weeks or months later. To provide a sense of scale a typical human head hair is approximately 100 µm thick. Therefore three coats applied over a surface area of 5.5 m² needs approximately 2.75 litres of polish, i.e., (5.5 m² X 3 coats)/ 6 m² per litre = 2.75 litres."
I seldom use the sanding sealer provided by the manufacturers under pre-catalysed lacquer. I use the top coats as their own sealers. Sanding sealers are convenient in a mass production environment but they tend to be a bit softer than the top coats. If the job is big enough I use gloss to seal followed by one coat of the sheen required. I generally prefer to get the finish on in two wet applications at about 125 to 140 µm each, unless the wood is open textured. For these I usually apply a coat with extra thinner in it (about 15-20%) at about 50-75 µm thick to seal the wood fibres and to prevent air bubbles backing out of the open pores which will spoil the two subsequent coats applied at about 100 µm each.
In response to your second question I generally add between 5% and 10% lacquer thinner to pre-cat lacquers. If it's very hot and dry I sometimes add a little lacquer retarder. The amount of thinner required depends on your spray set-up and the viscosity of the finish coating straight out of the can. In my case I use a low pressure turbine delivery system connected to an Asturo HVLP gun with a 1.2 mm needle and nozzle.
The polish manufacturers can tell you the requisite viscosity for spraying their polish if need be, and the recommended needle and nozzle size, and you can test viscosity with a viscosity cup. I don't use a viscosity cup nowadays because I can generally tell if the stuff is the right viscosity for spraying by watching how it runs off a stick-- a steady unbroken stream is what I look for, not thick and gloopy, nor thin, broken and dribbly. Slainte.
