you're hunting under an identical environment to that which was used to 'test' or 'sample' the various targets you might search for.
It doesn't matter if it is one of the Garrett's you used, some other brand, or your new White's MXT. ALL metal detector responses are affected by the target metal as well as any good and/or bad nearby target metal, as well as the nature of the ground environment they are located in.
Under IDEAL conditions, such as in an "air test" or with a desired coin positioned 'flat' to the coil and located shallowly in something like clean, dry wooschips or neutral sand, you will be able to 'test sample' some targets and then anticipate somewhat similar visual TID readings in a clean and friendly environment. One that is also masking-target free.
The MXT has a relative descriptive 'range' located under the TID display. Accurate TID readings will be achieve when the coil is swept with the target centered with the coil, and when the coin-type target is positioned 'flat' to the coil and not too deep. White's VDI numeric readout is perhaps the best in the industry ... however, it is only as good as the information received, and that's governed by target size, target shape, target position, target depth, metal make-up, outside issues such as RF interference, challenging ground environment, nearby trash (or good target) that can mask the signal, and we can't forget operator coil selection and coil control and control set-up.
If a target is in an odd position, a little too deep for best recognition, worn or damaged, too small to generate ample signal for good TID, located in highly mineralized ground or in a touch ground contour situation, or of the detector is hampered by interference or otherwise not set for peak performance and the coil sweep improper, you will likely end up with a visual TID that is 'off' from what you might anticipate. Let's not forget nearby masking targets or multiple target conditions.
Here's a challenge to learn what your detector can and can't do well:
