Jim, so far, I've been able to use a touch less than 50% thinners to paint, using white spirit as the thinners. Note that so far, I've only used one colour (Dark Earth), and originally thinned it so that it flowed down the mixing jar like milk, as is usual practice. Although this sprayed fine, it was very 'wet', ran and 'pooled' easily, and, of course, gave a thin coverage.
The next colour I'll be using (RAF Dark Green) I'll thin at around 40% thinners, and if it struggles, add a bit more thinners, up to 50% max.
As a rough guide, use 50/50 mix, and adjust as required, after test-spraying.