I think formulae are limited to 256 characters! Your solution is to either convert the formula to VBA code (but you would have to run it to refresh the custom field). Alternatively you are going to have to simplify the formula and break it into 4 or 5 different custom text fields that either return NA or teh answer. A further field will then return the result of any of the earlier fields not displaying NA. Personally I prefer VBA as it would be much easier to maintain and update.