jhurditch
A mid cruise weight could be anything, so many variables due to passenger loads and fuel loading that any figure given would be arbitrary.
A simple rule of thumb would be a basic weight of 18,000kg, fuel anything from lets say 2,000kg to 4,000kg (you did say mid-cruise, and assuming we've burned some off already), and then a rule of thumb for passenger/baggage weights we use to guestimate what weight we'd be when tanking fuel would be for every 10 passengers you would have on board 1,000kg.
So for a flight with 50 passengers on board, fuel remaining of 3,000kg and a basic empty weight of 18,000kg you would have a cruise loading of
18,000kg
50/10 x 1,000 = 5,000kg
3,000kg
Total weight = 26,000kg
Hope this helps