Seems I've figured out my focusing issues on my l900. I use the knurled ring around the lens to set focus at a given height, then the enlarger does the work when moving the head up and down. It holds focus beautifully now ! I don't know what the knob on the right of the head is for.....

http://youtu.be/FPtl8OIliYY

That is my enlarger...feel free to take a peek and let me know any hints you might have.

Thanks!