It would be great if one could transform a TO-DO into an event by just moving it with a mouse from the TO-DO list into the event list (and vice versa). Events and TO-DOs are already mutually transformable in the manager. Items in schedule views can already be resized with a mouse. Also, TO-DOs could be added to the calendar by being drag & dropped into any schedule window (day, week or month view).
That way, I would finally have my methodology covered by Rainlendar.
I build a biggish to-do list first, estimate each item's duration and finally assign time slots to them.