I've had my own share of mishaps with alumamark, it's a really delicate product to engrave, but once I got it engraving alright it's been very popular with the clientelle. I took the advice from the brochure that came with it when i bought it from johnson plastics and it worked just fine for me with the ULS 25e running at 55pwr and 80spd 500dpi.

Bieb you should check this very interesting post about true focus distance;
http://engravingetc.org/forum/index.php?webtag=EE&msg=1672.5