I've been cutting and engraving a few items lately. I find it to be highly experimental to find the correct speed and power settings for any given material. I did find a useful section of this manual
http://www.hltlasers.com/reference/25-50PSRevisionB1.pdf that gives suggested speed and power for cutting or engraving various items. This discussion starts on page 90. All speed settings are %speed for their laser which had a 25"/second max speed.
Perhaps we should post this info somewhere as a quick reference.
I thought about keeping a notebook for proper power and speeds of items, but then I decided to simple glue a small piece of the material to my wall and I write on the wall below it to record the speed and power setting for that cut.