You may be able to change this field to a text, number or date field (match to the result type specified for your calculation) and select your calculation as an auto-enter calculation.
Another option is to add another field to your database where you can manually enter or select data. Your calculation field then incorporates an If or Case function that returns the value of this added field if it is not empty, but returns the calculated value if it is empty. You can even place a transparent copy of this field on top of your calculation field and use conditional formatting or Hide Object When to hide the calculation field when the data entry field is not empty.
I'll try one of the above.