Various parameters types will accept mathematical equations as valid data. This means you can do some "math" within a parameter instead of figuring what the value you need is elsewhere and then enter it into Revit. To do so you must enter the equal sign (=) first followed by the formula.
As an example: I have three items that need to fit within an available space that is 12 feet wide. I'd like the middle item to be centered between the other two. Yes, I could add a dimension between just the three elements and use the EQ control. But less obvious perhaps and the reason for this post is that I can enter a calculation into the dimension field instead.
Formula is this: =((12'-5')-2.5')/2
In plain english:
Overall Space minus two Desk Widths minus one Desk Width divided by two
Yes it is easier in this case to do this instead...just discussing possibilities.