I found the answer to my own question.
Basically its my inexperience with the Set Field command.
The calculation should have been seperated by a ;
So it should have looked like this:
Set Field [Inspection_Invoice_Totals::ItemAmount;$ThisFieldContents]
The reason it didn't was I was placing the value portion of set field right in with the specified field. I needed to click the calculation button and enter the $ThisFieldContents in the calculation area.
Weird thing was Filemaker let me do this without it seeing an error until I ran the debugger and caught the error 5, basically no calculation.
Hope it can help someone!
The Set Field step takes 2 arguments:The Target field and The Calculated result.
Set Field [Inspector_Invoice_Totals::ItemAmount ; $ThisFieldContents]
You appear to have put the variable name in the "Repetition" entry when selecting the target field instead of specifying the calculated result.