You can use the default behavior for your measure on Organization and add a measure 'Estimated Order Count' which is a count on your fact table and a calculation for your time dimension:
Create Member CurrentCube.[Measures].[Average Estimated]
As [Measures].[Estimated amount]
/
[Measures].[Estimated Order Count],
Format_String = "Currency",
Associated_Measure_Group = 'Estimated' ;
Yann Nguyen France
MCITP: Business Intelligence Developer 2008