RTFM, jackass  ;-)

--How to determine the exposure compensation value:

1. After focusing the lens, read the extension
amount through the bellows extension
scale (A) on the top of the distance scale.
2. Find the compensation value by the
"Bellows extension scale/Exposure compensation
value" located on the right side
of the close-up photography table. For
example, assume that 127mm lens is focused
after combining it with No. 2 auto extension
tube. If the extension amount reads
35mm by the bellows extension amount
scale, it is understood that the compensation
value is + 2 steps by the scale located
on the right side of the close-up photography
table. In this case, increase exposure
by setting the shutter speed dial two
steps slower or by opening the aperture by
two steps.

<< Is there a quick formula
for any and all extension tubes?>>

Not truly a formual, but a table

(Extension / focal length) = Increase in f/stop

1/10 to 1/5 = 0.5 f/stop more
1/4 to 1/3 = 1
1/2 = 1.5
1 = 2
1.5 = 2.5
2 = 3

There's the formulae and all that, but I photocopied a ruler-type scale
from an old book that does the job. You add all the tubes and extension
you need, then bring the scale into view. Aligning one edge with the
edge of the frame, you read the number of the line that falls at the
other frame edge. This gives a direct correction factor.