The eternal problem - buy too little food and your guests leave hungry, buy too much and you're stuck eating it for a week after the event.
Do you have a feel for whether the people coming tend to be hearty eaters? If so bump it up a bit. Half a pound of lunch meat per person, plus whatever else you're serving, seems like it should be more than enough. There does come a point where you have to say that the focus is on the gathering even if there are a couple of people who are prone to seriously pig out given the chance.
If there's enough food for everyone to eat reasonably well without constantly worrying whether they are taking too much I'd say you'll be fine. If you have a 600lb relative who can't have a 17th sandwich because you ran out of meat then so be it. If you have people who didn't get a single sandwich because you ran out of meat that's a problem. Somewhere in the middle is the happy medium, which will partly be based on how much you want to spend on the meat and how you personally balance the desire to make sure everyone has enough with the desire to not be stuck eating bologna sandwiches for the rest of the year.