Is there any customization or rules to automatically set "To Do" value automatically after entering "Actual" hours?
Unless you wanted to make your own custom task management app there's nothing that would handle this. Actuals is a legacy field that's hidden by default and doesn't impact the Task Estimate or To Do fields. We used it briefly to evaluate the accuracy of our estimating rather than continually changing the Task Estimate when we ran over on a task, but we rapidly matured out of the need for it. Hope that helps.