You are measuring though the extension tubes and the lens, if the lens is stopped down the camera will show you what shutterspeed you need for a correct exposure, it is that simple.
On Auto the camera will choose the correct shutterspeed, no calculations or corrections needed as long as your object has a general density, so not (pure) white or (pure) black.
Peter and nsurit,
Thank you both for responding.
OK, I'm reassured and have been taking a few shots: seems just too easy!
I'll let you know how I got on, when the prints come back (still a few left to go on the roll).