Addback Calculator: this formula is responsible for calculating the approximate dosage of nutrients needed to be back up to regular levels.
This is not my formula, however brilliant, and I don't remember who/where it came from, only that I had it written down for reference. So mad props to the source.
((target – current) / target) * _ ml per gal * _ gal reservoir = dosage
target = what you want your ppm to be
current = what your ppm is at currently
_ ml per gal = whatever feed dosage you're using
and so on
You would do this for each separate nutrient. Hope this helps. Also, don't forget about pH.