The diameter of a circle determines all its characteristics.
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When dealing with pipe the volume is more important, therefore, I would use the inside diameter.
2*1673 = 3346 [don't forget that units are important].
20 refers to the amplification, 70 mm to the diameter of the main lens or mirror. Note: This diameter is by far the most important piece of information.
You do a diameter circle!!
It is half of the diameter or radius = diameter/2