I will assume, since it is a diffusion enlarger that there is some sort
of diffusion panel in the head, like frosted glass, white plexi, etc.
On most of these enlargers you must use the correct size enlarging bulb
as the center of the bulb must be in the focal point of the parabola
that is the head of the enlarger. For this reason, most of these
enlargers have a provision for adjusting the bulb up or down in the
head, which you do with the enlarger turned on but no neg in it. Move
the bulb up or down until you get the most even illumination. This
should get you close, but if it is not close enough, you need to make
the enlarger "fix itself" by making a graduated mask on a piece of film
as big as the diffusion plate in the head. Measure the unevenness on
the baseboard (the difference between the center and the corner of the
frame). Lets assume it is 1 stop. Expose a piece of film so that there
is almost no density on the corner, and develop it so that there is 0.3
density more in the center. This mask then is placed on top of the
diffusion panel in the head and acts as a graduated filter to even out
the light when it hits the baseboard.
Good luck
John
-- ÿWPCÛ