I know my colleague Zane has been in contact with you about this question. The suggested way of calculating Progress % of the Test set would be as follows
[Measures].[Xray Tests executed]
/
-- count of all created tests in a Test Set
DefaultContext((
[Xray Test Plan].CurrentMember,
[Xray Test Set].CurrentMember,
[Xray Test].CurrentMember,
[Measures].[Xray Tests created]
))