When growing hydro, a ppm meter can be helpful to monitor the RISE in ppm's as the plants grow.
Simply following the nutrient makers instructions work,,,,,,,to a point.
An easy way to run hydro is to use the 33% rule. You don't need a ppm meter for that!
Make a batch of nutrient solution to fill your res. at 100% of the makers suggested ratio.
As the res. needs to be "topped off". Add back a nutrient solution mixed at 33% ( mix up a gallon at 100% and add 2 gallons of straight ph'ed water = 33% nutrient solution) to top the res. off.
Do this UNTIL the amount of solution you will have added back equals the TOTAL amount the res. would hold.
Now dump the res. and refill with 100% nutrient solution and repeat the 33% top off.