If you start with an empty black tank, and then used the tank wand for a couple minutes, it should only leave a couple gallons behind. Not a big deal and you can probably live with it for the start of your next trip. As mentioned, you can time how long you use it on your tank, then take it outside with a 5 gallon bucket and run it in a 5 gallon bucket for the same amount of time and you will have your answer.
I used to have to use the tank wand to keep my sensors accurate (...I get annoyed when the tank is empty and it reads 1/3 or 2/3 full). I also sometime let family/friends use my RVs and I don't want them to wonder about the tank status (...or I don't want to wonder when they return the RV). I always hated getting out the tank wand plus hose, feeding it through a window and through the bedroom and to the bathroom (and then back out after it was used in the toilet/black tank). Messy and inconvenient. I solved this problem and made tank cleaning 10x easier and faster....I added a Tornado Tank Flush. Total cost was less than $50. Here is a link to the DIY project I did. Now when I dump I just spend 2 minutes at the dumping station using the flush with the black valve open and I can see tons more waste get cleared out after it was emptied once. It also keeps my sensors clean.
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/30315748.cfm
Hope that helps!
Chris
