Hi there, wondering what is prompting the need to have a custom form field for filling in time spent instead of the users logging time in the native WF methods?
If you add 'actual hours' to a project's task view, it'll pull logged hours for each task, which automatically sums with each parent task to easily see how many hrs were logged with each parent task (summation of all its child tasks' logged hours). I feel like that's your best bet...
If this helped you, please mark correct to help others : )