for my upcoming 4430 project I'd need two ports of 104 mm internal diameter and 210 mm length. Tubes of this diameter appear to be unobtainable here in Germany. Next standard diameters would be 96 mm or 121 mm.
How would I calculate the correct length with other diameters? Does the cylindrical volume count, i.e. longer ports of smaller diamaters and vice versa?

Best regards!