The DMB has this documented here: https://wiki.ubuntu.com/DeveloperMembershipBoard/KnowledgeBase#Quorum
My understanding is that since we require a absolute majority to pass a motion, we can pass motions with >50% of members present if we are unanimously in favour. If we aren’t unanimously in favour, then we fall back to having to get enough +1 votes such that the absent members would not be able to swing the vote below a simple majority even if they were to all vote -1.
In other words, quorum doesn’t really exist as a formal concept for us. We simply need an absolute majority to pass a motion, and quorum is our informal way of describing what we need to achieve that in a single meeting with some members absent.
For the avoidance of doubt: by absolute majority I mean that the requirement is that >50% of all DMB members are in favour, whether present or not.
As an aside, we have agreed (by absolute majority) that some types of decision can be made by only one member agreeing, but that’s not really relevant to the quorum question here.