I am the one who coded this tool and is still being updated frequently.
18% overstocking isn't that serious. As long as you are aware of this and you are keeping up with your water changes (which you appear to be doing), you are going to be fine. I had two tanks with 130%+ stocking according to http://aqasvisor.com
and I kept them safe and spawning for years. Of course I also had to do larger (or more frequent) water changes than other tanks. many mbuna tanks are by design, overstocked.
One of the point of writing this tool was to get people to get some sense in the bioload they have in their tanks. As someone else mentioned, what this tool suggest is a guideline and you learn and act accordingly. The most stupid thing to do is to say "Oh this tool says I am 150% stocked, that's BS. I've been doing 20% water change per week and they are doing fine."
I wouldn't worry about your current bioload. :)