you know bro,i dont believe there is just one method to our madness here,there might be one you enjoy more than another,but all have there place,so i dont know if you were around back in the summer when i was doing the 33 gal trash can hydro outside in triple digit heats,growing watermelons,so with that said,i mainly grow in soil,majority actually,but also like coco and now the hydro,will be doing all outdoors this year ,so there is no you need more experience and should have shit with me,i learn every damn day,so im pass something on to you that just might help your journey ,no matter how stupid it might seems,it works and why it works is because there is no more guess work,just like you just said you cant depend on manufactures of these nutes.
so that said here goes ,even if you dont aprove at least read it and think on it
you have say a three part nutrient feed schedule,so 3 bottles
take 3 glasses and fill with water all the same volume all the same ph ppm.
so you have 3 exact glasses,then take 1ml of each one of those bottles,and put in each glass,so you have all three nutes all in there on seperate glass,check the ph and ppm write it down what each one did.
now you have those numbers you know that when you fill and fresh nutes in your res is right ,when you top off which i was doing 5gal every 2 days in the can grow,i would check my ppm and ph first,i was taking the food to drop my ph and using potsassium bicarbonate to raise the ph when i gave the food it added the minerals back in and the bicarbonate help a ton on the swing a ton ,so you think unbalance,probally so but when i step back and look at plant and know that my res is right being topped and such and see a little yellowing,i think cal-mag so i ad the mag sulfate and so on,you know what the minerals you use does and work the res that way,33 gal is a lot to change every week and plant is on a tressil hanging on side of a wall growing melons,so no moving,thats what i came up with,in other words way outside the box worked for me.
so know what each product does ,you them for ph up down or whatever,read your plant and you cant got wrong bro