It’s simple. A round Tube with this measures 34x26x4MM means that it has a thickness of 4mm, the external diameter is 34mm and the internal diameter is 26mm
A = 34mm
B = 26mm
C = 4mm
The dimensions doesn’t have to be exact, but make sure that the drill fits perfectly inside of the tube, which means that the Internal dimension (B) is the most important to consider.