Hey Chuxtable
It seems I missed the first few lines of code when I pasted it into the message. Sorry about that. Hopefully it resolves your issue. 
Sub SumFunction  
  Dim objControl, objPage, GWDepth, WaterColumnThickness   
    Set objControl = ThisEvent.Object 
    Set objPage = ThisEvent.Object.Parent   
      GWDepth = objControl.Value  
      WaterColumnThickness = objPage.Controls("txtWell_Dep_TOC").Value - GWDepth
      objPage.Controls("txtWat_Column").Value = WaterColumnThickness
End Sub
I have also added some additional code to populate a few more fields on my form. Hopefully you can use these as well.
Sub SumFunction  
  Dim objControl, objPage, GWDepth, WaterColumnThickness, GroundwaterElevation, CasingVolume
    
    Set objControl = ThisEvent.Object 
    Set objPage = ThisEvent.Object.Parent   
      'Calculates water column thickness
      GWDepth = objControl.Value  
      WaterColumnThickness = objPage.Controls("txtWell_Dep_TOC").Value - GWDepth
      objPage.Controls("txtWat_Column").Value = WaterColumnThickness
      'Calculates groundwater elevation
      GWDepth = objControl.Value  
      GroundwaterElevation = objPage.Controls("txtWellElevation").Value - GWDepth
      objPage.Controls("txtGWElevation").Value = GroundwaterElevation
      'Calculates casing volume based on well casing diameter
      GWDepth = objControl.Value
      If objPage.Controls("txtCasDiameter").Value = 1 then 
      CasingVolume = objPage.Controls("txtWat_Column").Value * 0.04
      Elseif objPage.Controls("txtCasDiameter").Value = 2 then
      CasingVolume = objPage.Controls("txtWat_Column").Value * 0.16
      Elseif objPage.Controls("txtCasDiameter").Value = 4 then
      CasingVolume = objPage.Controls("txtWat_Column").Value * 0.65
      Elseif objPage.Controls("txtCasDiameter").Value = 6 then
      CasingVolume = objPage.Controls("txtWat_Column").Value * 1.47
      Elseif objPage.Controls("txtCasDiameter").Value = 8 then
      CasingVolume = objPage.Controls("txtWat_Column").Value * 10.1
      End if
      objPage.Controls("txtCasVolume").Value = CasingVolume
      
End Sub
These are all run in the onchange event in my GW Depth control
later,
JC