This worked sell to change a drawing dimension from imperialdisplay to metric (MM) display (Thank you SolidAir!)

Dim swSelMgr As SldWorks.SelectionMgr
Dim swDisplayDim As SldWorks.DisplayDimension
Dim Part As ModelDoc2

Set Part = swApp.ActiveDoc
...
...
...
If Me.optMetricYes.Value = True Then
Set swSelMgr = Part.SelectionManager
RetVal = Part.SelectByID("RD2@Drawing View7", "DIMENSION", 0, 0,0)
Set swDisplayDim = swSelMgr.GetSelectedObject6(1, -1)
RetVal = swDisplayDim.SetUnits(False, swMM, swDECIMAL, 3, True)
End If

BUT silly me I thought the "3" in
RetVal = swDisplayDim.SetUnits(False, swMM, swDECIMAL, 3, True)
would give me three decimal places but it does not. What is theright way to do it?
Thanx,
plh
SolidworksApi macros