The Goal Seek command 
automates engineering calculations based on a single variable to achieve a specific design goal.
Many calculations based on geometry can be solved in more than one way. You can change the Goal, the Target, and the Variable to achieve different goals. All of the goal seeking variables and values are specified on the Goal Seek command bar.
You can extend the length of time the goal seeking operation runs by editing the limits set on the Goal Seek Options dialog box.
The Goal Seek command operates on driven and driving formulas, variables, and dimensions attached to 2D and 3D geometry. The type of dimension you add or the variable that you select--driving or driven--determines what you can solve for.
Physical properties of a model and PMI dimensions are accessible as driven variables through the Variable Table.
Driven variables and dimensions, which are dependent, can only be selected for the Goal.
Example:
When you create an area object on 2D geometry, you generate driven variables for perimeter, area, and moments of inertia in the Variable Table. These can be selected as the Goal.
When you create a 3D part, you generate volume and surface area properties in the Physical Properties dialog box. When you assign a material in the Material Table, you generate mass properties based on the density of the material.
The units of the Target value must match the units of the selected Goal variable.
Driving variables and dimensions can only be selected for the Variable to be changed.
The Variable should be related to the Goal so that when a change is made to the Variable, it causes the Goal to change through a formulaic or geometric relationship.
As goal seeking finds a solution that matches the Target value, the dimensioned geometry updates to show the results of the calculation. The Variable Table and the Physical Properties dialog box are updated with the calculated values.
For more information, see Best practices in goal seeking.
Some things you can do to enhance the basic goal seeking workflow include the following:
Achieve different goals by changing the Goal, the Target, or the designated Variable to be changed.
Solve calculations in more than one way. You can continue to generate additional results by extending the duration that goal seeking runs. You can do this by changing the settings on the Goal Seek Options dialog box.
Annotate the design using callouts and property text to reference current values in the Variable Table and display the values in the callout. To learn how to do this, see the Help topic, Extract Variable Table data using property text.