Adjusting exposure for teleconverters and extension tubes

Warning: interim version! Only works for extension tubes when lens is (more or less) focussed at infinity.

To use the calculator, ... "NaN" is Javascript for "Not-a-number", and means someone has done something silly.

Aperture setting of lens f /
Extension tubes
(Enter lens focal length and total extension of tubes)
Lens f =
Effective f-number for exposure purposes f /

And is this correct? Is it accurate?

Yeah, of course it is, silly.