Be glad to Dutch! You take a milk bottle cap or something of similar size and fill it to the top with table salt. Then you add DISTILLED water (not RO or spring or tap) to the salt until it's completely filled.
Put that in a sealed bag or container with your meters, and leave it on the counter for at least 12 hours. The meter should read 75%. Mark it with whatever it does read so you know what the variation is.
Put that in a sealed bag or container with your meters, and leave it on the counter for at least 12 hours. The meter should read 75%. Mark it with whatever it does read so you know what the variation is.