How do I get sheet numbers into the files names on my prints using print organizer?

I'm trying to setup a named expression to, among other things, pull the sheet number from the plotted model and it's not working for me. I'm using the PrintDefinition.SheetNumber symbol but it only returns a value of "0" regardless of what number is set in the sheet number field of the model. Am I missing something or not understanding what that symbol is supposed to do? 

IIf(System.String.Length(PrintDefinition.ParentFolderName)=1,System.String.Format ("{0:D3}", PrintDefinition.SheetNumber),System.String.Format ("{0:D2}", PrintDefinition.SheetNumber))

Here is the expression I have setup. It is using the parent folder name for a prefix and then setting the number of places for the sheet number based on if that prefix is only one letter or more. I just can't get it to actually pull the number from the sheet number in the model. If I use ActiveModel.SheetNumber it will find the number, but then it puts the same sheet number from the active model on all the names instead of the number that goes with the corresponding model. 

I'm using V8i SS4 version.