@UkuleleGod:
Just one more quick question, then I'm off to do moooooore sanding (will it ever end...?):
Am I correct to measure the port length through the "centre" of the pipe, as the blue line shows in the graphic below?



Question is because somewhere I read that one should measure "the shortest way of air" which would not be the centre, but rather "from one corner to the opposite, and so on to the next one". It doesn't sound logical to me, I reckon my way is correct.