The way I do it is find the seam that runs up the drum, make a mark 18" on both sides of the seam (near the bottom), this will give you your side hole marks.
Measure 18" from the marks you just made, yes you will find out that they will over lap.
Now split the difference between the over lapping marks and that is the center or your front hole location.
No what I do is measure 2" from the bottom lip of the barrel and pop my intake holes.
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