FlowField based on code

I am disappointed to find that a FlowField cannot use a small amount of CAL code.
I am trying to calculate a substring of a UK Postcode on the Customer table, e.g.: AB12 3CD -> AB12
I have done this outside NAV in Excel reports by: PostcodeDistrict = Delete3CharsFromTheEnd(StripSpaces(Postcode))

What is the best way to do this in NAV to act just like a FlowField? In other words:

• Is not stored in the database
• Is available on the customer page
• Is available on the web service

Thanks!
(For anyone wondering, I did it this way because the first part is not a fixed length.)