How to create a customizable column in Sharepoint

I want to create a customizable column where a value can be passed by linking another column (default setting) but can be overwritten manually.

Example: For every month of a year there’s a column. For every month there’s a fixed budget that doesn’t change in standard cases. Sometimes it might happen that there a changes for a single month so it’s necessary to change that single value of that column manually.

When I choose a calculated value as default value I receive an error message because of a reference to another column. When I choose to only have values linked to another column I can’t do any manual changes afterwards.

Is there any trick to resolve this problem?

Solution

Use a workflow. Here is the concept:

create a new list item. Leave the month values blank, but put a value into a key field
run a workflow that triggers when a new item is created
let the workflow copy the value from the key field into the months

Now the month values have a variable amount pre-entered and they can be changed if required.