I used to be a stickler for religiously following the part-bowl with a new pipe 'rule'.
Not so much nowadays though(I don't buy a new pipe very often anyway), I just tend to load and pack them with a proper sized 'fill'.
For cake building, I keep them a little 'dirty' to help in that respect. When I've finished a bowl, I stir what's left with my nail, put my thumb over the bowl and give it a good shake. I tip out the resulting loose ashy-mess, clear the stem bore & draught hole and set aside ready for the next time. I find that by employing this particular routine, that I now actually have cake developing in most of my bowls. I never used to have any success in obtaining even the beginnings of a cake layer by following the part-bowl with a new pipe 'rule'.
I have never bought into that whole idea. I understand the logic, but I just don't buy it. Make it easy on yourself'; pack that pipe and smoke it. I have more than one pipe I have had for some 40 years and they seem to be working out just fine for me. I think that might be here for the long haul and I have never once "broken one in."
I always fill it to the top and smoke it to the bottom when smoking a new pipe. When I am done, I let the pipe cool over night. I use a couple of fluffy pipe cleaners doubled over and gently wipe the inside of the bowl, getting the ash and loose tobacco out. I always break in my new pipes with some type of flake tobacco and it builds a nice hard cake after a dozen bowls or so.