Project management formula Help for prepopulating dates.

What I'm trying to do is create a formula that will prefill down from a start date. The variables I have is my start date and hours
(listed by a work day hours/8). I want my start date for my next task to add one but I also want to have the formula take the previous day if it is lets say <= half and the next day task is <= than half to be on the same day. the problem I keep running into is that I will have multiple days that with half day tasks so I end up with multiple items staying on the same day, more than just 2. I know there is software that can do PM gannt charts and such but because I like playing with Excel I choose this way. Thank you I'm trying to leave my sample out since its work related.