Take a piece of paper,wrap it round the tube,mark a line on the paper where both edges meet,cut paper at this point.
Measure the paper for a accurate centerline.
Mark a point on the tube anywhere with a pencil or center punch,with one edge of the paper lined with the center punch marking and wrap the paper round the tube,the centerline on the paper is half the tubes circumference so mark it on the other side of the tube where it is.
Simple, basic but very good if the right tools are not available.
I'd use the same method as pete but with a scribe and stand.
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But how do you know if the hole is perfectly inline with the otherside?