Hopefully this isnt too stupid a question. If I am taking a macro/ close up shot and using a filter, do I apply the bellows extension compensation, treat this as my exposure proper, then apply the filter factor - or do i treat it like I am using two filters (ie mutiply the factors)?

Add them. The "filter factors" deal with "stops" of exposure, so are inherently NOT linear. Extension of the bellows actually results in "new" f/ stops - Larger ratios than those marked on the lens. The true value is indicated by the proper label: "f/" - referring to the formula [ f/x= aperture diameter ]; where "f" represents the distance from the aperture itself, wherever it is, to the film plane (not infinity focal length of the lens). As an example, if a 100mm lens that is marked as f/4 (arbitrarily calculated at the infinity focal length) is used when extended to 200mm for a close-up, the true f/stop is then numerically greater than what is marked, -- the aperture diameter is still 25mm (100/4) but the aperture-to-film distance has doubled to 200mm - therefore, 200/25 = f/8. I hope this helps. If not, let me know and I'll take another crack at explaining it.

Leon you ask a valid question. The answer is that you apply the factors sequentially. In other words, let's say that you were shooting your macro shot with a filter having a filter factor of 1 1/2 stops and that in this instance you are exposing your object through a 210 mm lens at F32 and you find that your bellows extension (from lens nodal point to film plane is 11 inches) the film in this instance is ISO 100. You meter the scene and the meter indicates an exposure of 1/4 second at that film speed and aperture without allowances for bellows extension or filter factor. When we take into consideration the filter factor your new exposure is 3/4 second (one stop is 1/2--the additional 1/2 stop is 1/4 second) at F32. We then apply the bellows extension factor of one stop (infinity focus is 8 1/4 inches and the bellows is 11 inches this is one stop) and your corrected exposure is 1 1/2 second. You are now into reciprocity consideration territory (over 1 second exposure). Typically this would now become a 3 second exposure at F32 with all of the factors included. When developing this sheet of film you will want to decrease development by 10% appr. to compensate for the increased contrast that the longer exposure for reciprocity considerations create. Hope that this explanation is helpful to your future calculations. Good luck.

So the filter factor gets applied after the exposure is adjusted for the extension. (or vice-versa) Thats great, thanks to you both for your help