Could you explain what your needs are?
Depending on the sense of priorities you want the calculation to have:
- Create a second field, 'DataOverride'
- Make the calulation field check this first, to see if there is data entered in it.
- The IF statement results in the DataOverride field if there is anything in it, and the related field if there is not.
- Put both fields on the layout and make them look identical
- Put the over-ride field behind the calculation field
- Make the calculation field non-enterable in Browse mode, and the DataOverride field enterable.
If there is no related record the field(s) will look blank.
If you click in 'the field' it will click through to the DataOverride field and let you (apparently) enter data in a calculation field.
another possibility depending on how important it is that the field remain a calculation field would be to change the field to a text or number field (as appropriate) and auto-enter the calculation you need. then you could always modify the value in it.