It is interesting to me how many engineers hop from English ("3 inches to go on the rubber band") to the metric system ("400 meters per second"). Andy Weir touches on this in his book "Hail Mary".
I am from NZ and find myself doing this. The difference is whether something is estimated or measured. If it’s a rough estimate, and the value doesn’t matter at all, I use inches. So “six inches” is more of a way of describing “about this much”. It’s deliberately imprecise.