Better than a thouie, they actually push out more lumens (yeah, I know, but it's still photons).
I haven't found run-off measurements to be helpful, I really do prefer the slurry test. That said, there's no need in my experience to wait hours to get an accurate set of results, a half hour all should be done.
Gather samples from different areas of each pot, mix those together. Add water, 0EC is best, be sure to measure the pH too, enough water to make a loose slurry. Wait 7-10mins, then measure EC and pH.