It's not an easy question to answer. For bicycle lugs the clearance gap will vary a bit if you use cast lugs, stamped lugs, change the angles etc. You can also close the gap a bit by tapping the lug while it's hot.

I recommend checking the series of articles "The Metallurgy of Brazing" by M. Emiliani in Bike Tech 1982. Part 3 checks the shear strength of joints vs. clearance. But then again you won't see framebuilders probing a socket with an ID gauge.

So let's say uh 0.3mm?