Yeah, you need the meters.
Ec vales are: 0.8-1.3 for seedlings; 0.5-1.3 for clones; 1.3-1.7 under vegetative phase; 1.2-2 during flowering.
Here is a chart to convert Ec into ppm, the ppm is something that you shoukd find labeled in bought water.
https://manicbotanix.com/ec-to-ppm-conversion-chart-2/
And yes you can use bought water, i speak this from experience with bottled water.