What you are seeking is a "bellows correction" of which you can find calclators and formula to find the extra aperture or time to add.
The amount varies with the focal length of the lens mounted on the tube.
This is quoted from a posting I made in Exposure Discussions- "Whats your coolest trick"

**Only real "trick" I can think of is figuring the approximate actual aperture with my calculator. Fer'instance 100mm len at f/8. Divide the focal length by aperture to get the aperture factor or 12.5 (which should be the real size of the aperture in mm). Measure the total extension, say its 150 mm from focal plane to lensboard. Divide that by the aperture factor to get 12. That tells me I'm really working with an aperture of f/12 for that magnification.
The math can be swapped around to figure what to set at a particular extension to get the working aperuter desired. Say the 150 mm total extension and the 100 mm lens, you want f/8. Divide 150 by 8 to get the factor of 18.75. Divide 100 by the factor 18.75 and you get 5.33. So you would set the aperture to a smidge wider than f/5.6 to get a working aperture of f/8.
It's worked every time I've used it. Only hard part is finding the damn calculator when I want it, it tends to turn invisible on me at times. **

In your case you will simply add the 28mm to the lenses focal length, and possibly add any amount of extension introduced when using the lenses focusing closer with its helicals and do the math.