Picked up a bargan-priced (but fine condition) telephoto lens, my first true telephoto lens for LF use, and have no idea how to calculate increases in exposure when shooting close-up with the lens. It is a Schneider-Kreuznach Tele-Xenar 240mm f5.5. Any help would be appreciated!

I suppose it doesn't matter that it's a telephoto lens. You should be able to quess or meassure the ratio between subject and picture size and add exposure according the ordinary formula.

Measure the length of the bellows when focused on your subject and compare it to the bellows length when focused on infinity. For a true telephoto lens you'll need to focus it at infinity and measure the bellows length at that focus, because the belows draw at infinity will be less than the actual focal length of the lens. Use the same units of measure for the bellows length ie: millimeters if your focal length is in millimeters or inches if your lens is measured in inches. Think of these numbers as f-stops and compare the difference in bellows draw from close-up to infinity. For example: If the bellows draw at infinity was 24 centimeters (240mm) and in a close-up focus the bellows draw is 64 centimeters (640mm), the difference is 2 stops, f24 (close to f22) compared to f64. Increase your exposure by 2 stops to account for the bellows factor. The key here is to measure the actual bellows draw at infinity focus for your "true" telephoto lens. A regular lens would focus at infinity at it's actual focal length.

Telephoto lenses are a special case. Yours should have a bellows extension of about 150mm when focused at infinity, but the rear node will still be at 240mm. So when extended to 250mm bellows, you will need ONE stop compensation, not two even if the bellows draw is nearly doubled: 250-150 = 100mm extra extension, 240+100 = 340mm real extension. Since 340/240 is 1.4, that gives one stop compensation. I know from experience that this is correct, although my tele lens is a 360/5.5 Tele-Xenar.

As I've said above, simply measure the difference in bellows draw from infinity to close-up focus. If your infinity bellows draw is 150mm and the close-up draw is 240mm, you have a difference of a little bit more than 1 stop. (f15 compared to f24). You can actually just measure the distance between the front and rear standards of your camera to make this correction. You're not interested in the actual focal length of the lens for this measurement, just the extra distance the light has to travel when that lens is focused on a close distance rather than at infinity. No need to make it harder than it is.

Brooks Short, as I've said before (though not in such cleear terms) you are wrong. The location of the lensboard has nothing to with the mathematics of the bellows compensation. but is merely a convenient reference point which happens to be fairly close to the nodal point of most lenses. But it is the nodal point that should be used for the calculation - and for many tele lenses it's actually outside the lens. Do a simple experiment: Mount a telephoto lens, measure extension at infinity. Then focus it to 1:1, measure the extension. Chech the reproduction ratio by direct measurement on the groundglass.

A tele requires more compensation for given subject size than normal and a retrofocus less. When I had one, I put the flat diffuser on the incident meter and with the camera focused on infinity, took a reading. I extended the bellows by the rear standard, measued the light and bellows distance extension about six times. You can remove the glass for this as you need to have it aimed on a white board that you do not move. I never had trouble with bellows extension again with that lens.