Hi Ben, you are closer to the answer than you think! This can easily be achieved with global variables. What you need to do is first to open the modify box on a dimension and type =x. Once you start to type after the equal sign a globe icon turns on. Click on this icon to create a new global variable named x.
Then you give it a value to start with, and select ok.
The global variable "x" is now in the equation folder in the featuremanager, and the dimension with equation sign is linked to it.
Link all the dimensions that should be the same value as "x" to the new global variable by typing =x or select the variable from the drop-down list.
Now we can change the value of x to change all dimensions at the same time. right-click the variable and select manage equation.
Change value and see the change on the model instantly (if automatically rebuild is on).
The global variables can be used in all sketches and most of the features in a part. For more info about equations and variables check out the help topic:2016 SOLIDWORKS Help - Equations