I do not have a "barrel break-in procedure" as such. I just shoot the rifle and once you have around 60 rounds down the tube you have got all you are going to get out of the rifle. I also do not clean until it stops grouping. You are going to get a lot of different opinions here. Do whatever you think will work best for you.
You are correct in saying all barrels are different, IMO the only true way to determine when YOUR barrel is done speeding up is to shoot through a chronograph to determine when your load has leveled off.
I usually see around 150 as well. My latest benchmark was right at 140 according to my notes when it finally leveled off. I know one very prominent builder who has many world record holding builds who doesnt fine tune a load until at least 200 rounds are down the tube.
The last couple it seemed like were still speeding up in the low 300 round count but these aren’t magnums. And it’s not big jumps just gradually creeping a little faster.