I'd start with a ten yard zero and see what your trajectory looks like at 15, 20, 25.
I zeroed the Steyr at 11 yards (10M) as that flattened it out a bit between 10 and 20 (i.e. less hold-under at 15 yards)
Chairgun is really useful for playing with alternate zero points once you've worked out the drop on the range.
Chances are if the targets aren't stupidly small you can aim dead on from 10-20 yards.