Eric, don't get caught up in worrying about the notch size of your trowel, and keep your eye on the goal of achieving as close as possible to 100 percent coverage on the back of each tile with a minimum thickness of 3/32nds of an inch when they're set. The absolute minimum requirement under industry standards is 80 percent coverage with good coverage under all edges and corners.
If your tiles are flat, and your substrate is flat, your 1/4"x3/8ths" notch might be more than adequate. You'll wanna be keying in mortar to the back of each tile anyway, and that will help. If you can't get the required coverage with what you've got, you might need to adjust your technique or your trowel, or both.
The coverage is what's important, not what you used to get it.
My opinion; worth price charged.