data validation-creating a costing model

Please can anyone help me with a data validation problem? I am creating a costing model where the user makes a variety of selections from pull down lists I have created in data validation.
In one section the user chooses a carton type of G1, G4 or other using list in cell A. When the user selects G1 or G4, cell B populates (via lookup) with no. of rolls in that size carton. The user cannot overtype this figures. If the user selects 'other', cell B defaults to zero and user is instructed to enter roll qty into cell C. The user can only enter into cell C if cell B is zero.
This appears to work okay except where the user firsts selects 'other' in cell A, enters value in cell C and then changes carton type in cell A to G1 or G4. The qty for G1/G4 automatically updates AND we have the manual entry for the 'other' carton type. Without having to rely on user to know that they need to delete the 'other' qty how can I get cell C to automatically zero if qty in cell B. At the moment the data validation in cell c is as follows

=IF(Q29=0,TRUE).

i think the easiest solution would be to Conditional Format the C cells such that they turn red if both B and C values are greater than zero and let the user enter zero in C. I don't know if you can zero C without circluar reference errors.