I proclaim myself "impressed!"

Kidding aside, I have found that $14 tach to the super helpful at getting the linkages and idle set right on my 220D. I had a sawing idle which was resolved by getting the linkages set at their correct lengths.

I vaguely recall somewhere in the FSM that if you don't have access to a tach, they suggest lowering the idle rpm until the engine stops running smoothly or until the alternator (generator) light comes on. The idle adjustment, as mentioned above, is a screw on the inlet butterfly valve. Super easy to do. You also want to be sure that your idle adjust cable is not set too tight. Even if you have backed it off all the way inside the car, if it is set wrong in the engine compartment, it could still be causing an elevated idle.

Good luck.

__________________
1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).