https://milwaukeeinstruments.com/milwaukee-cd611-digital-conductivity-pen-ec-/
at bottom it says to use one of the 2 primary scales... 500 or 700.
13 is likely 1.3 mS/cm, so 650ppm or 910ppm. This is why EC pens aren't so good at measuring ppm. It's usually good enough for context of gardening. Better off calculating from gauranteed analysis labels.
'gauranteed analysis' just means a gauranteed minimum not gauranteed accuracy, but still way closer to reality than an EC pen. Plent of free apps and sites will do this math for you.
Soil: how much came in the soil? Start on lower side and up the dose if you see deficiencies.
soiless/hydro: 100ppm to start and observe and adjust from there.
regardless, some trial and error will be necessary and there are many variables that imapct exactly how much Ca you need to keep the plant healhty. The pH you choose and concentrations of other nutrient molecules impact this.