I don't have one but wanted to get one. I have read alot on them and , yes i think you should be going with the 500. I have a hanana meter and that's what it told me to go by. If you just started and your plants are not used to it just take out half the rez and put RO water back in to bring it down and PH again. When I have mine in veg they get about 500 to 600 PPM's. Then when i go to flower I jump it up to 1200 PPM's and then additives brings it up close to 1600 or 1800. I am now in week 6 of flowering and my nutes and add ons are over 2000 PPM's. Some think that half nutes work just as good but I find when I pump the nutes to them they do much better. Some strains can't take high nute levels. Best way to tell is the tips of the leaves. If they start to burn on the tips, cut back. That's the good thing about hydro is you can correct things fast. I just put more WW and papaya in and just changed to what you have for PPM's. Have you tried going to their site and see what info they have on the meters ?