I had this same exact question!
different brands of ph up and down are more powerfull than others.
also I pretty sure the nutes raise the PPM, and thus takes more to raise the PH than just plain water.
add a 1/3 gal of water, mix one nutrient, stir thoughly, test ph, adjust accordingly, add another 1/3 of the gallon of water, stir test adjust if needed add the next nutrient in your line up, add the last 1/3 of your gallon test/adjust ect. you get idea?
thats what I'm gonna try myself next feeding, thanks to Stumps for that tip.