I'm not worried
T, I wasn't questioning your experience, just noting that the numbers didn't equate to the specific problem. There isn't anything wrong with being careful and if it is easy and gives piece of mind to beef things up then it's all good but this type of question arises fairly regularily and for most tanks of that size there is no problem.
I can't recall the last time I've heard of a floor collapsing because of a non custom aquarium actually. I'm sure it has happened somewhere but if it was that close a call I'm sure there would be more incidents.
This week I have serviced a 230G., 300G., 150G., 140G. and a 135G. tanks; all on joisted floors - no collapses so far.