do you plan to screw an adapter to that for pressurizing thru the intake ?
the intake/reed cage are the most common offenders for leaks, but pressurizing thru the spark plug does not test that, IF the reeds even close tight enuf to seal against the back pressure ?
bottom line is ....pressurizing thru the intake is the best method, not the spark plug.
Also.....i'm not so certain how that pressure relief valve/button will work inline below the gauges ?
isn't that for holding highest pressure to the gauge during a compression test ? how will that allow leak off if you have a leak ?