If the MDF is 
proofgrade draftboard, there is a default setting for it.
If it’s random MDF I’d start there, and run tests (check out #6) - or reach out to the seller for settings if it was sold as laser safe.
If it’s from Home Despot or the like it might not cut at all, a lot of MDF is made with glues that a laser can’t cut.
5 Likes