I never bother measuring run off pH, as long as I get good run off it will automatically renew/refresh itself. So I pH to 5.8 or above, usual 5.8 to 6.2. I don't use pH up after nutes, if needed I put it in first, then pH the whole solution back down. Putting in alkaline minerals, like pH up, into an already acidic solution will cause minerals to settle out.
The reason I don't go below 5.8 is the pH scale is logarithmic and each drop below 5.8 gets way harder to uptake calcium. Growing with LEDs I need all the calcium I can uptake.