G'day downunder.
Welcome to the forum.
Down to 8 inches, if a coin is flat to the coil, your TID will be accurate. After that or if it is near the metal, then this can throw your numbers off a bit. If I'm in this situation, I'll actually move the coil past it away from the ground, just to see where they show up on the meter. If you make a mental note of that, while hunting closeish to the metal, then quite often, when you pass over a coin, the detector can average the reading out between the 2 targets. The idea, is to simply be aware that your numbers can be off a bit. it's best to listen to how the signal sounds when you go over it. I'm a bit rusty at the moment, both with hunting around play equipment (not much opportunity here in Dubbo) and I have played with my X-Terra a huge amount lately. I've been using a few other detectors.
If a coin is on edge, then it can give what sounds like a short rapid hit. Play around with a coin over the coil at different angles and see what happens. I love the X-Terra's audio as it gives you a pretty good idea of what is under the coil without having to consult with the meter. It's not perfect, but good enough. Generally, if you are getting a good steady reading or only bounces a bit, then a target is worth looking at. You'll figure out the common trash pretty quick and can avoid those numbers, if there isn't normally good targets in that range.It will just depend on the place you are hunting.
If you have any specific questions that you'd like to ask, then fire away. If I can't help you on a specific point then I'll tell you, but there are plenty of experienced and helpful folk here that can also help out.
Don't be a stranger. Being able to talk to other like minded folk here really adds a great dimension to the hobby and can add some personal satisfaction by being able to contribute. Just remember, that by asking questions, you are contributing in a positive way. Other folks that may not have though about what you asked or may not have known, will learn something new. I have learned heaps from reading questions that other folks have asked and answered. Also, it's a great way to share experiences and have a few laughs on the way.
All the best.
Mick Evans.