A simple way to calculate exposure compensation is to use a ruler. I keep a small tape measure with me for this purpose.

Follow along:

1. In your mind convert your len's focal length to inches.

Examples:

90mm = 3.5"
150mm = 6"
210mm = 8.25"
300mm = 12"

2. Now focus any one of your lenses at infinity It will be it's mm length from the film plane. I use the film plane and the center of the front standard to measure from.

3. In your mind convert the mm focal length to inches.

Round it off to the closest f/number.
In your mind call the resulting number an f/stop.
For example my 210mm lens is 8.25" which is my f/8 lens.
My 90mm lens is 3.5" which is my f/4 lens.
150mm lens is 5.9" which is my f/5.6 lens.
300mm lens is my f/11 lens (close enough to 12")

4. When you are focused for your photograph use the tape measure to measure the distance from the film plane to the front standard reference point.

Examples:

Lets say for example that you are using a 210mm lens and it is focused at 11 inches. 210mm = "f/8" and my "f/8 lens" at 11' is f/11. f/8 to f/11 = One stop bellows compensation.

Fast and accurate. Try it.