Hi Dave,
Sure thing. You can do this with Rules or with Code. For the Rules approach double click on the type selection and go to Rules. Define a rule to set a field's value (in this case the revised total field). Then, you can use XPath to sum the values in your table and multiple by 0.75 or whatever. You may also want to add the same rule to values in the table since they could change after the type is selected.
For Code, you can add OnAfterChange handlers with code to do the same calculation and set the total fields based on it.
Hope this helps!
Patrick Halstead [InfoPath MVP]
InfoPathDev
Patrick Halstead
Project Manager at Qdabra