I think it was more about being able to understand/read what the measurement devices were showing. The exact distance probably wasn't as important as the criticality could also vary with other variables (e.g. geometry).

As in, you're trying to understand the situation as "shouldn't they have precisely nailed down all the parameters, if the goal is to measure when X starts happening". But it seems Slotin was more demonstrating "this is what you're going to see on the monitors when stuff is close to going boom". It wasn't about "this specific distance is a safe gap" and more "here's how you can tell whether the gap is safe".

He was about to be reassigned out of the lab, and was demonstrating equipment to his designated successors.