Does your ppm pen also do cf or ec? as this is more accuate than the 500/700 scale.As just looked at Dyno-Gro feeding chart and they using the 700 ppm scale.
500-Scale
To get an EC value, multiply the ppm reading by 2 and divide by 1000. Thus, if your EC is 1: 1 * 1000/2= 500 ppm.
700-scale
To get an EC value, divide the ppm by 700 (1680ppm / 700 = 2.4EC)
10 CF = 1 EC
* CF divided by 10 = EC
* EC multiplied by 10 = CF