Hi there,
To be honest with you, and I don't know if other guys in here agree, but my personal experience, I usually dilute the flat with a bit of tamiya acrylic thinner, and then I spray at moderate pressure, so that I get an even cover, but also a strongish flow, it is a bit of trial and error, but thinning definitely helps with the flow. Varnishes are quite thick to start with.
All the best
GB