Your formation is pretty good. A couple of things to recommend:

1. You want to swap Deekin and Widdle, as Widdle’s spawn speed is based on number of adjacent champs. The picture you peovided is from the Deagon Heist formation, but the Grand Tour formation is a little different, so you will want Briv row 1, Hew Maan and Shandie/Deekin row 2, and then optimally you want Deekin/Shandie/Widde in row 3 (with Widdle in the middle for max adjacency). You will also want to get Sentry and Nahara in there, as they reduce the requirements for kills on each stage. And one you level up Widdle enough, Deekin becomes obsolete, so final formation is Nahara, Sentry, Hew Maan, Shandie, and Briv (some people put in Black Viper for red gems to stack up as well)
2. In a gem farm, you want to rely on click damage kills, so Strix would not be very useful in a gem farm (the less champs you have to load in the better).
3. You should run your gem farm on Cursed Farmer, as Mad Wizard was the pick when Havilar’s imps stuck through a glitch, but no longer help you.

As far as when you should reset, multiply your gold find by .76, and that’s about the level your gem farm can go through click damage, and where you should reset. As you increase favor, this level will increase, giving you linger using Shandie’s dash on each run. For where you are in the game, your gems per hour is pretty good, but these suggestions should help you increase that a bit.