Are you looking to default the auto-calculation to always Medium or something else?
The Customization Guide tells you where the decision tree is located and how to modify that. For version 7.0 its on page 21 and on 7.1 SP2 its on page 30 but basically the decision tree is in the SD.Feeder.GeneralIncidentSubmitForm for the end-user or client-facing form and then SD.Feeder.TechnicianIncidentForms for the technicians.
Hope this helps or points you in the right direction.