So presumably you have worked around 17 days at 900 boxes a day. So you have loaded 15,300 boxes approximately, with two misloads you have have a misload rate of 1 box per 7650. Keep up the good work.
In our building the "allowed" misload rate is 1 per 4000. of course this is always changing, when I started working in 07, it was at 1 per 2000, then it crept up to i per 4000 over a few years.
If management is giving you guff about your numbers, ask what other peoples are, I am sure you are doing better than most people with your misload rate. dont try to please them, they will never be satisfied with what you do.