It's pretty clear from the game specific drawings with some trig.
http://www.usfirst.org/sites/default...c_Drawings.pdf
The game manual under Arena 2.2.5:
http://www.usfirst.org/sites/default...c_Drawings.pdf
Has a centerline at 30", 60", 90"
If your robot can't deal with the difference in height between the bar being at 30" to the top or 30" to the middle, you need to reconsider your design. Field variation alone could affect that height.