General Hydroponics pH Control Kit

This is the stuff you want. The indicator you can buy alone for CHEAP. But you need up/down too.

You read right about flushing. 3 x volume.

Soil in the tube isn't nearly as accurate as testing the RUNOFF. That's standard practice. You get a far better profile of what overall conditions throughout the pot are, what the roots are seeing, you know? Anything that is available including undesirable salts contributes to the pH this way. Plus soil from the surface of the pot is never indicative of conditions deep withing the pot, where root activity is and there is less oxygen, so chemicals could potentially exist in different forms.