Your math is correct. Just to be sure, you should get some more weights as already stated. But they will likely confirm what you already know.
Your WD does appear to be transferring the proper amount of weight.
The main concern here is that weights can shift in the course of a trip. Your 9% TW could possibly go DOWN!. This can get you in the danger area. This is why the general recomendation is 10-15%. I like to be on the heavy side of that.
The only reason for the 15% limitation is the limits of the hitch and TV... My stuff is strong enough to go past that.
As you know, going too low on TW can induce sway. This won't matter how strong the TV hitch is.
Too low is too low.
