The screencam tut in this post
shows basic workflow of creating an expression to link a property to another and using the PWS to minimize the amount of typing you have to do.
In the tut I stopped with making one property equal to another. But if I had a situation where I wanted the property to be equal to half of the other property I could have manually added
to the end of the expression. An asterisk means "multiply".
The tut is missing some sound, but the essential portion is there.