I used a piece of paper with 5 mm squares in the neg holder, put the enlarger head in its top position, measured the sides of the "neg" on the baseboard and adjusted the fixing of the enlarger pole until the opposite sides were equal.

BTW, talking about old enlargers. I use a Meopta Axomat II with a Belar 4,5/50 (4 aperture blades). It is sharp enough for me (I seldom do larger than 24 x 30 cm).