No offense taken here, but I think you are confusing MBS with WLL or SWL. The MBS for the tubular webbing you are using with one water knot is about 4340lbs or 19.31kN. So a typical 5:1 WLL/SWL on 4340lbs will be 868lbs (32lbs less than the Petzl Z). Add more knots and friction gear to your webbing and that number decreases. Your carabiner also has a WLL, which is probably a factor of 1/4th or 1/5th of its MBS too. The manufacturer specs should state what their WLL rating is. Its important to understand what all these numbers and ratings mean.