Metadata properties - automatically set KM document properties

Hi,

I'm trying to set custom properties based on other custom properties. I understand how the dependOn works for the "Allowed Values" and "Additional Metadata", but it does not quite cover my needs.

Say I select one value from a dropdown list - I want the next property to be set to a fixed value the user can not edit. This second property should be updated if another value is selected from the dropdown list. There is not a 1-to-1 reference between the two properties, more of a n-to-1.

Would the easiest thing be to write a service that would update the document properties?

Hope someone understands my needs, and has the answer to them.