I've never heard of a dental implant, interesting!
If she is having problems chewing properly due to the tooth issue, don't feed hay. Switch to soaked hay cubes or soaked pellets instead.
Can't answer about your flake of hay, because all flakes are different. You can weigh your flake, but even then, it's hard because each flake, even from the same bale, are different. I don't weigh my full size horse's hay on a scale, but I do go by feel. None of them are indanger of being overweight, so I err on the side of more is better, so I don't feel I NEED to weigh theirs.
But with my minis, since they ARE prone to getting overweight, we DID weigh their feed. They get soaked pellets, soaked beet pulp, and some get Jr and/or Sr. We weighed it out in the beginning, and know how much of a scoop (because we weighed it) to feed them. We also give them bermuda hay, but our bermuda here isn't very good nutritionally, so we give it to give them chewing time and fiber. We don't weigh the hay for them since it doesn't do much for their weight. But other hay DOES, so you can't go by that like we can ;) We make sure their nutrition comes from the pellets.