This is certainly not a simple request for a lot of applications. You have to ask yourself how you want to control the variable.
You can capture any measurement analysis as a feature and use its parameters in subsequent relations. So capturing the master is not a problem. But how do you drive the slave to equal the master? If you can create a formula to manage the slave, then you can balance that with the saved measurement of the master.
For instance. I have two rectangles. The master is 2x2. The area is 4. I save the area measurement of a "fill" feature. the slave is 3xn. The only variable is n. Your relation would be n=<parameter for area measurement>/<dimension symbol for "3">. Life would be good if all calculations were this simple 