Test the mix on a flat glass plate held vertical.... keep adding wee bits of thinner to the mix until the brush marks level out and 'flow'. Too much thinner and .... 'sags'; too little thinner and 'brush marks'.

Keep checking the 'plate' during the day as the surface and paint temps. change.
Not sure about "penetrol" with BM urethane, check with BM before you ruin a gallon or two ... but if compatible is one of the best 'levelers' for any oil based, etc. paint.

Benj. Moore is one of THE FAVORITES of my friends who are pro painters --- a full, undiluted real paint .... 'almost' as good as an old fashioned 'leaded paint'.

On any non-catalyzed ('cept lacquer) paint if you flat sand and then buff too quick before a total cure ..... 'gloss die back'. Wait a few days of longer for a better 'cure' before you flat sand .... and then come back a few WEEKS later and rebuff to restore more gloss.

Works on most paints and varnishes ..... other than infernal 'latex'. The exact thinner to paint ratio, matched to humidity and temp. is the 'secret', even when spraying.