Replacing mercury thermostat with wifi. I have just a red and white wire connected to current thermostat and boiler. There is a green present, but it isn't hooked to anything on boiler. So I can use the green if I can identify the common on the boiler. I think I've found the blue common, but would like to confirm. I believe I just need to connect the green wire to the wifi thermostat and to the bottom post of the first picture, Green would then be my common. Am I on the right track? My thermostat wiring is routed through several pumps, but final connection is here on the boiler.
Inside, a blue wire is connected to the bottom terminal of above picture (circled in blue on left), but there is also a red wire connected to that post. Circled on the right side is the blue wire connecting to post numbered 6.