Do not enter totals or subtotals as absolute values in the datasheet, but instead use an e (for equals). think-cell will calculate these segments according to the previous segments' values. If your totals consist of more than one stacked segment, you can enter an e for only one of them.
In addition, the Force This End Onto Baseline buttons should only be used when you are completely sure that the Waterfall diagram is correct.
Finally, you can move the handles of the connecting lines with the mouse to adjust the flow.