Silvie,
The formula for a refractor is: Half the length of the radius of aperture multiplied by the focal length of the radius of circumfrence, only if the f/ratio is less than 8 or greater than the diameter of the eyepiece variance. Then add the length of travel of the focuser to that of the diameter of the aperture and there you have it.
Now, a reflector is a different story. You have to double the aperture size, add the radius of the bolt in the EQ mount and use a spectrometer to check the colour of the tube. Once done, just halve it.
Glad to be of assistance.
