Hi, in "BSA twins & triples" Roy Bacon writes 0.010 in & out for A10 51-54, which is what I have been using (plunger, iron top). There is a mention for 0.015 in & out for 1950 A10, early production valves, top or what?? 0.010 in & 0.016 out is listed for A10 from 55.
I would not try going under 0.010, but it could work fine, who knows? A way of checking whats going on is to measure the result clearance when the engine is (very) hot, to be sure you still have some clearance on all four, and it's interesting to get a readout of how much the clearance have decreased from whatever you choosed to start with (edit: from cold engine, of course). BTW I have a clear conception that a little noise from the top is a good safety measure!
Hope this helps
E.