Definitely an example of why this sort of testing is as much art as science and you need a good baseline before the data is useful to anyone. In this case you'dknow the value is astronomical relative to plant h ealth and ignore or re-do the measurement.
The timing matters. early in runoff will be higher than late in runoff.
if you have a lot of ongoing recurring issues or doing something new, it's not a bad habit to have. if you run smooth grows, spot checking is enough.
I do soiless and religious about 10% runoff. I have properly ph-balanced nutrients. I used to check and then spot check the first year or 2 of growing. I stopped doing it and in 5+ years never had a pH drift problem or a runaway EC. Doing things the "right" way avoids it. What i put in is not what i expect the reading to be. I expect it to be consistent -- the resulting equilibrium of my fertilizer mix over time. As long as the 10% runoff is relgiious, it won't drift. Again, i do not expect it to be exactly what i put in.
In soil it's not so cut and dry, so you may have a greater need to keep track of this stuff, but form a baseline of "normal". Don't expect it to be what you put into the soil.
The only way to use such runoff testing is to know what "normal" looks like when your plants are supremely healthy and rocking out. that's the runoff values you want to maintain. This stuff can somtimes takes months to show a problem, so have a long memory when you set this baseline. From then on anything deviating too much is a problem.