Una, Fritz’s human, here. Have you always heard the rule “2-3 wrinkles” in regards to fitting your bridle and bit? That is what I was taught when I was a kid. While that rule definitely helps you get the bit in the right approximate location, be sure to always check the placement INSIDE of the mouth too.
I was having some issues with my gelding fighting the bit and everything looked good (from the outside). I had someone else check it and they thought it was good too. Well, apparently my horse only needs about 1 wrinkle from the outside to get the bit properly over the tongue, over the bars and between the teeth properly. So what was happening is the bit was on occasion bumping his premolars (it was adjusted too high in his mouth). My trainer helped show me how to check the inside of the mouth for proper fitment too instead of just relying on wrinkles!
You want to make sure the bit is in between the canine teeth and premolars. This way it sits above the tongue and bars properly. This will help you avoid any issues in your training.
If you have any questions, I would love to help you in any way possible!