Of course I cannot make any promises here for you, but I can tell you that your pick-up for tour will mostly likely be between 20:45 and 21:15.
Some good things to know (you are reading an FAQ, hopefully this page is full of them) are that often this flight seems to be on schedule within ten minutes. However, understand that airlines today (especially with smaller aircraft like the ones that operate the flights into Yellowknife) do keep their aircraft on tight turn schedules so delays to that aircraft through the day can become accumulative. And remember if you're travelling in winter, de-icing is not uncommon in Canada, which can add extra time to your journey. 
If you staying at a hotel that offers a free shuttle, consider instead taking a taxi ($20, and they will be waiting outside the terminal) to save time. Obviously you can save a lot of time by taking just a carry-on so you aren't waiting for your bag to be delivered at the baggage carrousel too. If you decide to book and you leave me a note in your reservation, I will make you my last pick-up to give you as much time as possible.
Ultimately, if you can not make your tour - even to the airlines fault - I still have to charge you, but if I have empty seats on a coming night, I'm happy to move you over to those.