(I got your PM)
I haven't tried the backside method. My engine comes out every two years or so and that when I do mine.
I use this method but it's very difficult if not impossible to do it with the engine in the car. There's just no room for the Gauge.
Try the conventional method using something like this. (Available from our host)
1978 Porsche 911 SC Coupe - Cylinder Head & Valvetrain - Page 4