It isn't the Round function that's not working properly. If you're using the
.Range.Text of the bookmark and the text is hidden, the value of that
property is returned as the empty string, "". Passing that to the Val
function, explicitly or implicitly, returns the number 0. That's what is
being passed to the Round function.

The solution is to set the TextRetrievalMode.IncludeHiddenText property of
the range to True, so it sees the text regardless of whether it's hidden.
Here's a sample:

Sub demo()
   Dim myVal As Single
   Dim oRg As Range

   Set oRg = ActiveDocument.Bookmarks("bk1").Range
   oRg.TextRetrievalMode.IncludeHiddenText = True
   myVal = Round(Val(oRg.Text), 2)
   MsgBox myVal
End Sub

Hi Thomas,

To reference the hidden bookmark's value, you need to code your field like:
{=ROUND({REF Bookmark \* Charformat}*1/6,-3) \# ,0;-,0}
By embedding a REF field in your formula with the '\* Charformat' switch, you bypass the bookmarked text's hidden attribute - just
make sure the 'R' in 'REF' isn't formatted as hidden.

Cheers