Larry Clinton At Frankfort, (Central) Indiana
I doubt there is a practical "thinned limit", unless you spread the material so thin the curing could not take place. I doubt this would be a "practical" mix. Bill Tindall may be able to estimate what that would be. But as you thin you apply a thinner film, therefore you would have to apply many coats to build the film thickness.
I have sprayed several coats of Behlen Qualalacq lacquer thinned below 50% (counting the solvent and retarder together) and have had no issues. But it did take a few coats to obtain the thickness I wanted.