Another fellow here,Andy, sent me some old fork crowns and steerer tubes for my practice sessions.
So I took an old fork and cut off the blades to braze into the crown. I notice that the gag is quite large but it did fill with brass. But how much of a gap is acceptable?

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?

0.03mm is about 0.011". Somewhere i read or was told that brass/bronze fit up gaps were 3X silvers (I assume this was 56%) and with silvers being about 0.003"-0.005" a 0.011" gap sounds close. But (there's frequently a "butt" in the room) fitting lugs and sockets is not a precise thing. Sure we try to hold the gaps to a minimum, tapping down the lug's edges is a well known technique, but stamped lugs will have tight and loose ID fit to the tube (that itself isn't always really round) at different points. Oh, are you modding the lug angles? Now you're way off the best fit ups. This is why I've generally described lug fit up with other than numerical references.

Slide the lug on the tube. Hold the tube vertical and watch the lug's ability to slide down the tube due to only gravity. A 56% silver fit up has the lug sticking in place but will slide a bit if the tube was tapped against a bench top. Not stuck but will stay put if not prodded. Brass/bronze will have the lug wanting to slide down the vertical tube on it's own weight or with minimal tapping of the tube on the bench top. The greater the length of the lug's fit up with the tube (say a classic MtB seat tube top sleeve) the tad more gap can be easier to flow through. Andy.

Here's an interesting chart/info based on research done back 100years ago. Notice this is butt joints with silver, not lap joints (ie lugs) but I think it gives at least some indicator of the dynamic going on with bigger gaps and how strength gradually drops away. The point I take away is that a tube fitting fairly snugly (.003" or less) is very strong and easier to make nice shorelines. I'm not saying .003 is what you should aim for but, that's a fairly snug fit and even tighter is ok it seems. I think something that tight for brass would prevent it from flowing but I don't know that.

The Famous Joint-Strength vs. Joint-Clearance Chart - Kay & Associates Brazing Consultants

I would be interested in hearing from Doug or Mark on the idea of putting a layer of nickel silver around the inside of the socket and then grinding to get the fit you want before brazing it up with 56% silver.