So, I finally got this figured out!
I knew that I was off 5/16" and that my (SystemParameters.FullPrimaryScreenWidth / 96) (to get the width in inches) resulted in a double value with decimals.
I determined the difference between the rounded screen width and the true screen width (with decimals).
I then multiplied that by the number of screen width inches (rounded) to get the total difference.
I finally multipled that by 6 since there are 6 pixels per 1/16".
Here's my code that determined how much was lost in the screen width rounding calculation.
Private Sub SetRulerPositionOffset() 
Dim screenWidthRounding As Double = ScreenWidthRounding() 

_rulerCenterXOffset = SystemParameters.FullPrimaryScreenWidth  ScrollViewer.HorizontalOffset  DesignCanvas.Margin.Left + screenWidthRounding 
_rulerCenterYOffset = SystemParameters.FullPrimaryScreenWidth  ScrollViewer.VerticalOffset  DesignCanvas.Margin.Top  screenWidthRounding 
End Sub 

Private Function ScreenWidthRounding() As Double 
Dim actualWidthInInches As Double = SystemParameters.FullPrimaryScreenWidth / 96 
Dim roundedWidthInInches As Double = Math.Round(SystemParameters.FullPrimaryScreenWidth / 96) 
Dim widthDifference As Double = roundedWidthInInches  actualWidthInInches 
Dim widthRounding As Double = widthDifference * roundedWidthInInches * 6 ' 6 pixels per 1/16 of an inch 
Return widthRounding 
End Function 
This doesn't seem a very eloquent solution but it works for me.
If there is another way to tackle this issue I'd sure like to hear about it.
Thanks,
Rita