Thanks for the replies everyone!
Rollei_Nut: Right, I forgot all about that A/M switch on the lens! The Fujinon lens I used doesn't have one, but I have a Takumar that does. I'll use that instead next time. The camera I used does have TTL meter, but I prefer using a handheld one for incident readings. Thanks!
Tony & Ian: Wow, thanks very much for providing the compensation needed. Those maths equations and formulas really boggled my mind! Haha. Anyway, I did a quick test just now with a 12-frame roll of film. Used both the 10mm & 17mm tubes together, mostly set at infinity. Shot quickly through the roll, developed it and from the negatives, I can see that the exposure compensation that the 2 of you provided does seem to be correct.
One other question: Do the compensation factors given change with the format & type of lens used? I.e I have another set of extension tubes coming for my Mamiya 645. Mean to use it with the 80/2.8. Do the factors given above still apply?