Recalculate calculated custom fields upon object completion

I need some custom form fields to be calculated actually at the very end of project process meaning right before it gets closed off.

Currently the task custom forms involving eg. Planned Hours do not recalculate if any changes occured after its creation.

I want these to be finally recalculated either upon Task or Project completion (preferable both).

Or at least to be able to mark such fields manually in the Custom form (recalculate upon related object completion - or something similar).