Based on what we've seen and samples we've paid for, it's a small percentage. They wish to remain anonymous, so we won't name names. Guess to yourself if you like, but please don't ask. The penetration is complete, with little or no spillover. It's hard to reach the highest level of skill in anything by doing it part time.
Concentrated repetition is a good teacher.
This is especially true of brazing, an art that is largely controlling molten metal with a flame, coercing it here, pulling it there, filling up the gap between tube and lug without spilling it out onto the tube, and taking minimal time to do it. Frame building is more than brazing, though. Every frame is a puzzle of dimensions that have to be kept, tolerances that have to be met, symmetry that must be maintained, placements that have to be to spec, joint penetration that must be thorough but not overflowing, and aesthetic judgment. Without the last item on the list, a frame can be true to the spec on the blueprint, but out of balance and even downright ugly. What one boy cares about, another might not.