XAML Numeric Format in TextBox

I have a UserControl to offer users a numeric up-down control.  I am now trying to figure out how to format the entered or incremented value properly.  Here is what I have so far:

<TextBox x:Name="txtValue" Style="{StaticResource ValueTextBox}" 
 Text="{Binding NumberValue, ElementName=Nud, StringFormat='{}{0:#0.00}'}" 
 FontFamily="Consolas" TextAlignment="Right" />

This works PERFECTLY using the StringFormat attribute, but as you can see, the format is hard-coded in the XAML.  The NumberValue DP that the text box is bound to is of type Double.

Instead of hard-coding my numeric format, I want to use a Dependency Property so the users can set that format at design-time when they use my control.

I added a DP called ValueFormat, which is of type String.  My intent is for the user to enter a format such as "#0.00" in that DP.  For this to work, I would have to replace the "#0.00" portion of the StringFormat attribute above with a binding. This I cannot figure out how to do.

I'd like to keep this in the XAML if possible.  Does anybody know of a way to put the string format into the DP without having to include the "{}{0:" and final "}" parts?

Thanks...

Ron Mittelman