Radio-button value from calculation and manually
I have a radio-buttom field specifying the route by which you can give a drug by: Stomach, intestine or IV. I can code the drugs 0 or 1, 1 = IV. I want the route field to automatically have the value 'IV' if the the drug is coded '1', anything else will be '"S" because most non-IV drugs go via the stomach. However, I'd like the user to be able to override an "S" route to convert it to "I" [intestine] where necessary but NOT be able to change "IV".
I'm not sure how to get both the automatic calculation entry and the potential override on the same visible field.