In my theory, max Exp Per Second possible inside the game using all available boosts to an average player, utilizing most optimally possible way of grinding, is 8k exp per second.
Adjust exp hack accordingly. If you'll use x4 and kill 1 average zombie per second, you should be fine. But if you'll start grinding, and receive more than 8k exp per second for like a minute or several, high chance of getting flagged for 24h autoban.