Your one to two mm of toe-in is what my trikes are set at. I use a dowel with a screw in one end which gives extreme accuracy and makes it easy to re-check from time to time. Also, there is some frame (and other parts) flex so someone should be on the trike when checking this setting. On a new trike I do the first check on the bench and do a final check while sitting on the trike.
Many disc brakes move only one side of the caliper and do, in fact, rely on disc flex for the braking action.
Chip