Sorry I dont have my giotto here to take a measurement mj.
I can however shed some light on the sudden steam pressure drop. Its not associated with the steam arm prob, but rather, the anti-vac valve that screws into the top of the boiler. Youll need to replace that (inexpensive) and wrap a little teflon tape around the thread before you do.
Its a common prob with the giotto. To reduce the likelihood of it occurring again I suggest you open the valve when you switch on your machine; wait for steam to start coming out of the arm, and then close the valve. When you switch off your machine you can open the valve again so that it will be ready for whenever you next turn it on.