MrStinkyanswered grow question 3 years ago I would think best way to figure out would be the following:
Check your volume, ppm and pH measure every 24 hours. You will see how much ppm & water the plants are going through every day and what the related pH change is.
If you have a very high base pH of your water supply and have to add lots of pH- to get to where you need to be after adding nutes, once the ppm has depleted you may experience a yoyo effect.
Along that line (from the other side) that if you fill up a 5L res to say 500ppm (as an example) and overnight your res loses 1L but the ppm is now 750ppm you need to dilute to bring it back as your nutes are effectively more concentrated due to the drop in your res and it is almost certain this would cause a pH swing too.
I would suggest diluting first to fill the res and then ppming to where you need to be, wait 5-10 mins to let it circulate and then pH it - you may find that you can plot a bit of a course over a week, figure out roughly how much water and how much ppm is used per day and make micro adjustments every so often for topping up res and adding nutes and pH - rather than trying to hold out for a week at a time.
Like rasta said below - air quality on your bubblers can effect pH too.