If you want a better estimate you need to bumb TC up a bit (64 or so) and then spam the button for a while. Then you will get values like 11700-12500 or something like that. I'm not suggesting that you calculate the average, just look at it for a while and you will get a feel for it.
What you absolutely can not do is what you just did. The variations are big enough that you could end up with a better range after 7 hours. Obviously you have not refueled even if the numbers seem to indicate that
