I'm unsure Steffen 'litmus test' as a real or decisively indicative test (OED) is the same as a 'ballpark figure' or 'gues(s)timate'- query: a ninepin bowl-park test....

The context of the question suggests a 'novus actus interveniens' approach. There are contributory factors, but the end-result remains the same and it is for judges, like the late English Judge Lord Denning, to apportion blame for the accident e.g. 60/40% or 70/30%,

Posted upon Phil's encouragement, but I am by no means sure if this really hits the mark ;-)

I would simply use "test" here. As in, "The test is:"

The tricky thing here is that an arithmetical procedure (Neunerprobe = casting out nines) is used in a metaphorical way, as it is in the following passage:
"Tag der Autonomie: Neunerjahre als Neunerprobe...Die Politik hat Lösungen durch Kompromiss erzielt, bei dem auf etwas verzichtet wurde, man jedoch gleichzeitig etwas Wichtiges erhalten habe: nämlich gegenseitiges Vertrauen“, sagte Historiker Carlo Romeo."
https://www.diebaz.com/2019/09/06/tag-der-autonomie-neunerja...

What we have here seems to be a process of elimination and litmus test at the same time. Unfortunately, 'casting out nines' seems to have less potential to become a figure of speech than its German counterpart.  Maybe it's not possible to come up with an English term that contains both ingredients. The best I was able to do was to come up with a workaround and transfer some of the meaning to the rest of the sentence.

Als Neunerprobe gilt: Denkt man die einzelnen Ursachen weg, so entfällt nicht der Gesamterfolg, sondern nur..
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Here's a handy test: If you set aside all individual factors, this shouldn't impact the overall result but only...