Retarder needs to be adjusted to the weather in the paint room. Humidity, temperature, barometric pressure (since Randy says so, sure he's right). I always do a practice board on which I shoot test patches. Sometimes testing a batch a few times. Especially when doing different coats during a given day. I use retarder to keep out the orange peel (bumps caused from drying too quick). Also, if the paint looks dull when applying, might indicate drying too quick. The aim is usually a wet looking coat. My mix batches tend to be small, a couple of ounces at most. Small amounts of retarder (a few splashes sometimes) is enough. Because I'm adjusting to conditions the mix may vary. I'm on the learning curve, have been doing spray paint for a few years. The feel for what needs to be done gets better. A conservative approach to retarder is probably the right way to go.