If it's a calculated field I can't change it's value.
Even if I add it to the query as Exp1: field I can't change it's value.
But if you want to change a value that has any persistence whatsoever, AND want it to have potentially different values for different records, you HAVE to make it bound, in which case the problem is solved - just not necessarily to your liking.
You see, a form that shows records in continuous mode holds NO DATA for each record. It is just a display template. At most you could put one value in an unbound field in the template, but the continuous mode of display means that template is re-used for every record. There is ONLY ONE LINE in the template. If you put data in the template, the data will be reused for every record using the template.
By way of comparison: If the form was not set up as continuous, then it would have been set up as single-record style, and there it wouldn't matter that you are putting something in an unbound template field. The mode of the form wouldn't replicate that field to other records because it would only show one at a time.
If you want to change the value for one record with the implication that you would NOT change the value for a different record, you are forced into a narrow path. If that "only affect one record from the continuous group" is the behavior you wanted, the underlying field has to exist in the record to remember the difference.
The only other alternative is if the continuous form is actually a sub-form and you could put a value in an unbound text box on the parent form and have it be applied when you select a particular record. But there, if you are dealing with the AfterUpdate event, you are stuck with the fact that the AfterUpdate event would be on the parent and that would imply that focus is on the parent, so you would have to have something in the subform to remember that it was selected, which implies another (checkbox) field anyway.