Hi Mark, assuming the plant sap you are sending to the lab is the same type of sample/same consistency as what you are putting in the Horiba meter, it might work to just do a simple conversion from ppm to percentage: 1 ppm = 0.0001%, or 1% = 10,000 ppm so your 0.39% lab test is 3900 ppm. That said, I don’t know the details of how the Horiba meter works relative to sap analysis so it may be impossible to directly compare results from the two methods with a simple conversion.