The same considerations affecting bellows extension that apply to view cameras would also apply to your macro bellows. There are several different methods of calculating exposure compensations.

One is to measure the relative size of the actual object as compared to the size of the image on the focusing screen.

Another is to measure the amount of bellows extension and adding that to the amount of lens nodal point to film plane dimension when only the lens is used. For instance if you are using a 105 mm lens (this for sake of conversation would be four inches) if you add four inches of bellows extension to the exposure (this would increase the first dimension by four and a total of 8 inches). In computing this you would convert the inches to Fstops so you have effectively increased it from F4 to F8 or a factor of two stops. So the proper exposure would be two stops more then your spot meter indicates. I personally would not rely on averages of meter readings. I would spot meter the low values and base my exposure on that.

If you have a metered prism on your Sq then the metered prism should automatically adjust the exposure for you.

Hope that this helps. Good luck.