Hydrukey aggro range

Does anybody know or have a general idea of the hydrukey aggro range? I was hunting with a friend and we spotted a few. Managed to kill two of them, but while we were reloading our rifles the third suddenly aggro'd and bum rushed us. We both got knocked out and survived the encounter, but I swear that we were too far to aggro. It's possible that it's slow ponderous movements lulled me into a false sense of security and allowed it to get close enough to aggro, but I'd like a rule of thumb for staying just outside it's range.