I have a custom module with a numeric field that is updated from time to time.

I want to add-up all the changes to field and present it in a different field as a rollup summery. For example:

number field = 100 --> updated to (-30)

so now I need the number field to show -30, but the rollup summery field to show 70 (the result of 100 + (-30)).

the most logical to me would be a formula field that adds itself with the new number field, but for some reason, you can't use the value of formula field in the formula itself.

Any ideas how to acheive this?

Thank you :)