First, SUM is not required if you are aggregating the data from lower level of hierarchy to the Highest Level. It will auto aggregate.
Second: In order to do the Top down approach we generally use LOOKUP. But you need to set a logic right as in how are you going to dis aggregate the data . For example If you have one Region and under that region you have got 3 areas
Let's say Region has got 100 as a value in it. Now you need to figure out how are you going to dis-aggregate this 100 into 3 areas. You can use Prior year actual's ratio or you can set up your own distribution ratio. In your area module you use LOOKUP functionality and multiply that with your ratio.
Or else if you simply want to pull down 100 in each Area use Simple LOOKUP on your Region and it will pull the value 100 in each area