There are two easy formulas.

The first uses the actual bellows draw of your camera, divided by the focal length of the lens you are using. The formula is X = BD/FL; exposure factor equals the bellows draw divided by the focal length of your lens. If you have the bellows racked out to about 30cm, and your lens is 21cm, then the formula is (30x30)/(21x21), which equals 900/441, which gives you 2.04; round that off to 2.0. You would have to give 2 times more exposure (one f/stop) to compensate for the bellows draw.

I prefer to use inches when calculating my exposure factors with this formula, as my main lens is a venerable 8-inch Kodak Ektar. You can use any units of measurement for these calculations, as long as you don't mix, say, inches and centimeters. Doing that will give you the quickest route to exposure disaster.

The second uses the magnification, which is the best one to use when you are photographing something small. First calculate the magnification, which is the object height (the size of the image of what you are photographing, as imaged on your ground glass (gg); measure it with a ruler) divided by the subject height (the actual size of the your subject). It is expressed mathematically as follows: M = h/h'. For example, if you have an image on your gg which is 4.5", and the original is 2.5", then M = 4.5/2.5 = 1.80.

Next, plug it into this formula X = (M + 1)(M + 1). That should read "M plus 1, squared." In this case, it would be X = (1.8 + 1)(1.8 + 1), which equals 7.84; round that off to 8.0. You would have to give 8 times more exposure (three f/stops) to compensate for the bellows draw.

I keep a folding ruler, a tape measure and an inexpensive calculator in my 4x5 kit; there are often calculators built into most cell phones, as well. And cameras, too; I won't go there, though...

BTW, if you are eight times your focal length, or more, away from your subject, you need not bother with an exposure compensation calculation; it won't be necessary. For your 210mm lens, that would be 210x8, or 1.68m, approximately 5.5 feet.