Screens

I'm a print shop owner who is doing the graphic design for a screen printer buddy of mine and I have a few questions. I'm use to designing for the printing process but not screen printing. I use an HP 5000n direct to plate laser printer to create my plates (or at least the plates that don't require a tight screen) I've used this same printer to print on Vellum so my buddy can burn his screens. This works fine for solids, but I'm having an issue with screens or gradients. The Printer resolutions I have available are:
106lpi / 300dpi
106lpi / 600dpi
180lpi / 1200dpi
60lpi / 300dpi
85lpi / 600dpi

Any help on what I should be using? Thanks

Location: 
United States

I would recomend leaving the dpi at default, and the output at about 37-45 lpi per color with either dot or ellipse pattern (the lower the lpi the larger the dot). Print to file to your rip software, and print your vellums through the rip program.

It depends what your buddy requires, 60lpi is what he would like (given the choices) but 300dpi/60lpi gives a Postscript cell 5x5 pixels. You can have a dot or not in each pixel space, therefore you have ONLY 25 shades of grey and you will see a band every 4% on your gradient. That will look horrible. Sorry.