The ribbon command Data | group Analyze | Goal seek 
can help you to determine what value a certain component of a calculation must meet to extract a predefined result from the calculation.
Note: This is done by mathematical approximation, and as with all approximation methods, the result cannot be guaranteed completely accurate.
Before executing this command, navigate to the cell that contains the calculation.
Upon executing the command, a dialog box appears. This is operated as follows:
▪In Formula cell, enter the address of the cell that contains the formula. (Generally this is preset by PlanMaker.)
▪In Target value, enter the desired result of the calculation.
▪In Variable cell, enter the cell address of the cell in which PlanMaker will test different values until the formula cell returns the desired result.
▪In Accuracy, you can specify how close the result must match the desired target value. PlanMaker will attempt to approximate values until the difference to the target value is smaller than Accuracy.
If PlanMaker is unable to find a sufficiently close result after some seconds, it will determine that the value cannot be calculated through approximation and will abort the search.
An example
In the worksheet shown below, the SUM function has been used in cell A5 to add the numbers 1, 2 and 3 in the cells A2, A3 and A4. The result in cell A5 is 6. The intent is to determine to which value cell A3 must be changed so that the result of the addition is 7.
For this purpose, move the cell frame to the cell with the calculation whose result is to be modified – in this case, cell A5 with the SUM formula – and apply the goal seek.
Notice in the figure how to fill in the values in the dialog box. When you click OK, PlanMaker starts the approximation process and displays the result 3 in a dialog box.
This means that the content of cell A3 must be changed from 2 to 3 so that the calculation has the result 7. If you click the Use button, the found result will be copied into cell A3.