I probably average re-barreling or building new rifles 4 times a yr. There is no set rd count for breakin, I have had a couple Brux barrels broke in in 15 rds, Bartlien's 200 rds. I've have had them setup in 1-3 shots, and others hover between up to a 60fps difference for 50 shots before settling down. Have noticed too, what was normal with say a Krieger 5 yrs ago, may be out the door today, hand lapping is getting better.
The last thing I am going to do is load 100 rds in anticipation of a breakin, may end up pulling half of them for being over pressure or inaccurate, no reason to waste barrel life here.
I do not believe in breaking in custom handlapped per see, but I will have cleaned the barrel 3 times by the time I have 70 rds down a new barrel. I usually just run a velocity check of what I plan on using, 10-16 shots, I will the borescope the barrel to see if there is a problematic spot or spots I need to concern myself with. Clean, shoot a modified ladder at distance, clean, find a satisfactory breakin load, clean, clean again once breakin is complete. Do final load dev.
Edit: Once I have a breakin load, the magneto hangs off the end till speed up is complete, I can bang steel with it on w/o much accuracy degradation.