make your mix, dilute a sample of the mix, oone part mix one part water, water ever that reading is, you double it and that is your reading for the mix. The sample will be HALF STRENGTH of the nute mix. PPM is a ratio not a defined amount....1 gallon of water tests the same as one cup, so for ever 1,000,000 parts of water, you want, say i dunno 1500 parts of nutrient mix.. well make your mix according to instructions, take a sample and dilute it so if it WAS 1500 PPM and you diluted it with the SAME AMOUNT OF water THEN you would read 750 PPM because the SAMPLE is DILUTED. because the sample is diluted by exactly HALF 1 part water 1 part nute mix, you Multiply by 2 to get your whole number again.
EDIT:
Sorry if I came off as a lil ignorant, I am on lunch break and in a hurry, and when I said "double it" I am only talking about the reading from your PPM meter, not the mix.