I work for JOBO Fototechnic, the USA distributor for Durst Enlargers, so please consider my comments accordingly.
With the Durst L 1200 Condenser head, there are three "dimensions" to lamp alignment. You have two of them on the left side of the lamphouse. One knob "focuses" the lamp moving it forward and backward within the lamphouse. The other knob tilts the lens in and out of the central axis of the condenser lenses. The third control is the lamp socket itself. You can move the lamp socket within the socket holder. This enables you extend or retract the lamp again to find the center of the lens axis.
As one of the other respondants has indicated, use of a light meter or enlarging meter can help you to be more objective in your adjustments. Make sure you have all the condensers in place and the appropriate enlarger lens mounted when you make the measurements. (Please note that the condenser model numbers i.e. Femocon 50, 80 or 151,152, represent the approximate enlarger lens focal length they are designed to work with. Don't try to use a 135 or 150mm enlarging lens with an 80 condenser. You will never avoid severe falloff of illumination.)
Measure the center of the illumination on the baseboard. Then move the meter out from the center in each direction. Notice which way the light fallof seems to be the worst, and then make adjustments to try and correct it. Once the lamp is centered, by moving its socket and the tilt control knob, then begin "focusing" the lamp. This should be the easiest approach.
I hope this helps.