Expression:Error: Calculated Column trying to convert to fraction instead of decimal

Hi lachie-1992 

The error is stating that "We cannot apply operator / to types Text and Numbers" so the first thing you can do is check and apply correct data types on your columns that you use for this calculation.

If this does not help, please provide a small data sample + screen prints, including both good records and bad records of base columns and Custom column with the error.