If you have "lost" the setting for the mixture, the best way I've found to get close is to adjust for maximum rpm, then ccw two or three clicks. This will get you close. You can then monitor fuel consumption and tailpipe condition -- if you are too rich still, the pipe will stay black and sooty, and your milage will be poor. Adjust one click lean and see what happens.
The range is very small, and the bad part is that milage will decrease too lean as well as too rich, since the fuel won't burn properly. If another click lean doesn't help, go a couple clicks rich instead.
However, unless you can get access to a CO meter, this is the best I've managed. It will run best at the rich end, though, so max rpm will be just a shade rich (about 3% CO, I think).
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!