The Oromo people would customarily plant a coffee tree on the graves of powerful sorcerers. They believed that the first coffee bush sprang up from the tears that the god of heaven shed over the corpse of a dead sorcerer.
As with just about all things coffee, James Hoffmann is a good starting point.
This should help dial in your grind size to reach a target brew time. Trial and error (with notes!) I'd half the fun to figure out your recipe. At the point my ratio/grind is pretty scalable for small and large pours.
Hard to tell without knowing your grinder.
I have an original ode, grind on a few steps before low, and usually 16-17gs of ground beans. Brewed in a plastic v60 with paper filters, this tends to be fantastic.
There's so many variables here that it's hard to give a good diagnostic. But mentioning the grinder and the brew method might help substantially.
Also, if you already found the sweetspot for the bag youre on don't question it. The quality will change from bag to bag, for this bag if you found 15gs to be what you're looking for then stick with it til the bags gone. Don't look a gift horse in the grill as it were.