ContextAction Mobile Button Editor is a plugin that makes it a lot easier to position ContextAction buttons. Normally you get the position using trial and error, but this plugin emulates the buttons so you know the right position you need for the buttons. You can change the position of the buttons by either dragging the buttons or modifying the ‘Position’ textbox. Here is a demo of how the plugin works:
I highly recommend that you use a device emulator while using this plugin to ensure that you have the most accurate results because the position and size of the buttons will vary from device to device.