The problem with testing runoff:
The soil is a random variable. You have the plant sucking in nutrients and water at different rates which changes the PH. You have microbial activity, 100s of different soil mixes, and many other factors.
Imo. PH the stuff going in and ignore the runoff.
And Btw, Soiless mixes, such as pro-mix and sunshine #4, are actually a pseudo hydro medium and should be PH'd lower than soil. I like to PH my nutes 6.0-6.2 in soiless. If not, you will limit the Phosphorus available in flower.