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.