Validating Value Lists

[Raater]

Hi

Not sure if this is an easy problem to solve?

I have a a field on a record which is a drop down value list. I want to ensure that one particular value 'Completed' can only be selected by users if the contents of the 'Client Account Number' match a particular customer and the 'Purchase Order' field is NOT empty otherwise 'Completed' cannot be selected.

Do I use validation by calculation or what are my options?

Many thanks for your help

Rich