Ignition measurement for angled spark plug

To measure where the ignition should be with a measurement tool is handy when the spark plug is positioned straight up in the cylinder head. BUT, how do you do that when the spark plug is angled? Of course, you have a look at this table that shows you how to translate the piston distance from the top when the spark plug is angled. The two examples (30 and 12 degrees) are from Sachs "standard" and the 50 S -engine.