https://www.cloudynights.com/topic/6...-newt-rebuild/
FL is measured to focal plane, which includes the 1/2 tube span from the 2ndary mirror and focuser height/focuser plate/tube thickness etc from the tube. The focal plane will sit slightly outside of the focuser, that's where you spend your time trying to get the ep's to sit for focus.
If the mirrors are filthy you may only be magnifying dust at the higher power?