Hey David,
There is probably some rhyme and reason about the holes sizes. Far be it for me to have those answers. I was just a copy cat on that. I just shortened the one in the picture I posted earlier. It has less than 20 holes, but they are just a bit larger that in my store bought for 35/40 engines. Maybe if DOC or Bob Reeves reads this they can shed some light on the numbers. Let me know how it works and how quiet it is. Actually here at our field, we are not required to run the mufflers.
Lee TGD