I usually don’t use the lift in the MPII building, but when I have a trolley with me (to get drinks for our working group) I of course use it. To minimize the average waiting time for a lift, I suggest to do the following:

Always send the lift back to the floor where you entered it.

On average, this should work pretty well. In the morning it would thus always be on the ground floor (when people are coming). In the evening it would be on one of the upper floors (when people are leaving). Even at other times it should work ok, as the arrivals/departures of people are not stochastically independent, i.e., if person A enters the lift on floor X, there’s a pretty good chance that soon person B will also want the lift on floor X.

To minimize the worst case waiting time it would of course be best to always send it to a floor in the middle.

If you have additional knowledge about the schedule in the building, you can also design a better strategy (sending it to certain floors at certain times). But this one is about as simple as it gets, but should still be very effective.