How to deal with incremental feed (feed gets cleared after query)

I'm working on a feed that is incremental and will include only properties which have been added or updated since the last run of the feed.

How should I deal with this? I tried with MigrateSourceMultiItems and tested what happens if I delete an item from xml. In admin I see -1 in the UNPROCESSED column. Now while the node was not deleted if it's removed from xml (which is good) then is this still the proper way of doing this? Just do a normal xml migrate and be ok with the unproccessed items cumulating?

Thank you!