It seems to me that input trim only matters much if you’re trying to match the intended / listed gain level of a capture with Gain at 5. Amalgam, for example, directs you to use 8.0 as a starting point for single coils and 0.0 as a starting point for humbuckers with their captures.
The first part is correct, the second part kinda but not really.
The GLOBAL input trim and the PER PRESET gain controls are two separate or combined ways to adjust your input signal level to get either 1) the intended gain of the capture or 2) whatever the heck you want. Remember, Tonex adjusts to different signal levels very well, more like a real amp than most digital users are probably used to, so you don't need to be married to the gain level as captured.
The suggestion from Karlis (Amalgam) has always been to follow the IK instructions for setting trim, which is basically to test with your highest output level hard strumming and adjust to be high but below clipping. The suggested values of about 8 and about 0 are just approximations that may get you in the ballpark depending on pickups, pickup height, strings, right hand technique etc.
If you mostly use it with one guitar, or all your guitars have similar output levels, I agree with the IK advice, and suggest setting the input trim to just below clipping with the hottest signal coming from your guitar/board. BUT if you have multiple guitar output levels, consider setting the input trim for the highest output guitar and then using the per-preset gain control to have different presets for lower output guitars.
Or, you can always allow it to have less gain for lower output pickups like the online mob pushing for calibrated levels would suggest. You know "if I plug in this guitar I want it to sound exactly like if I plugged into the real amp while Karlis was capturing it" crowd. The problem with that approach is it is not how anyone who knew what they were doing ever used a real amp! When you plug your guitar into a new amp, the first thing you would normally do is adjust the controls to get the volume, gain, and tome as you want it. No one ever plugs in thinking I want the amp set exactly the way the last guy left the knobs for his guitar because that is what should be perfect for my different guitar and playing style.