Can you say more about what that calculation is based on? Do you need to capture something from outside the form like time or are you using values in the form to compute it? If you're using values in the form, you may be able to use once(), trigger or a combination of the two to get the desired behavior. It sounds like you want to allow changes all the way up to finalization in Collect but guarantee no changes even if the source values change from a server edit, is that right? With a more concrete example we may be able to come up with an approach.