9990ppm? what the hell are u mixin up over there man??? lol i gota cheapo from ebay. i think it was like 10.00 with shipping and only reads up to 1999, but i bought 2 just incase. when i was useing RO water it would read as low as 5ppm.
and i may be wrong so let me know if i am, but when you have a starting PPM of say your 58PPM you divide that number in half and and that to your total PPMs to figure what nutes to add. example (58 divided by 2 is 29, so if you are shotting for 1000PPMs you mix to 1029PPM and that will give you dead on 1000PPM of nutes)
but really who is going to worry about that small of a PPM change.im useing 145PPM tap water to start my nutes, and havent noticed the plants doing any differently than the plants i grew in 5PPM RO water.