On the Edge 800 I used to hit the power button and then the power icon to get to the calibrate screen. The 520 requires about a dozen key presses to get to the same point. I think the 500 prompts you to calibrate if it detects a Powermeter.
Is there a shortcut to get to the calibrate screen? The manual mentions there are some shortcut menus which I didn't know about when riding. Not sure if they will help.