Oh, great use case. Anaplan has one exception that allows you to have both a formula and be editable for the same line item and that's with native versions. Use the formula scope - in this case I want Actuals to be editable and all other versions actual.
Another use case is when we want a formula for one version and a different formula for another version depending on the time horizon. For example, I want Actuals to be editable for elapsed periods (so I can load them in) and the forecast for unelapsed periods.