Consecutive Row Numbers?

I’m building an agenda for a workshop and want to achieve the same result as in this thread Cumulative Start Times for Event Schedule. The user would set the start time and then set duration per activity (row). The formula I need is to populate a Start Time column automatically for each activity (row) calculated on start time + duration of all previous activities (rows). The formula in linked thread above works as a solution, until the user moves a row to a new position in the table.