Since you are new to MDG, make sure to bookmark this document: Configuration and Enhancement of SAP Master Data Governance. Please make sure to look there first then do some research in the community before posting new questions.
I think the word you meant to use is "validation" not "valuation" which carries a different meaning. Any way, this document (found in the above link) should give you an idea: How to Check or Derive an Attribute Value in MDG using BRFPlus.