Sorry, you're going to have to pardon my ignorance............ What's a biscuit test? Is it as obvious as it sounds? :-)
Its spreading biscuits out over the cooking grate and running an average fire for 15 to 20 minutes. Will be able to tell from the doneness and appearance of the biscuits where there's hot spots. Some also use white bread. There should be plenty on YouTube, I can't think of a good one, right off.
From what you've said about the middle of your cooking grate being hotter than the stack end, I'm wondering if your smoker has a baffle on the exchange between the firebox and the cook chamber. You posted a pic above looking into the firebox and I think I see a baffle, but maybe not. A baffle will direct air/heat down and then it rises in the middle of the cooking grate and creates a hot spot.
Some smokers have small baffles, some don't have any. My Franklin smoker has a shelf like yours, which makes me wonder why the builder would put in a baffle. I can't imagine he did that.
The hottest part of my smoker is right next to that shelf. Aaron Franklin designed it so heat would rise immediately inside the cook chamber, then hit that shelf and start to turn and tumble, which creates convection on the stack end of the cooker.
Here's a pic of a biscuit test I did on my Old Country Brazos, that I owned before I got the Franklin . I've flipped the biscuits over because the bottoms tell more than the tops. I did this test when I was playing around with tuning plates, which in the end, was a waste of time.