The distance of your grow light depends on the type of light you’re using. If you don’t have a meter, the best check you can do is to stand under your lights at keep your body at canopy level. If the light or heat you are feeling on your skin after 5-10 minutes is uncomfortable to you, it will be for your plants too.
PAR meters are ideal but can be expensive for most growers. A LUX meter however can be purchased for under $25. If you’re using a PAR meter, at canopy levels we want between 600 and 900 micromoles in the vegetative stage and 800-1000 micromoles during flowering. If you’re using a LUX meter, the intensity will depend on the type of light you’re using. There are conversion factors that can be found here that will help you convert LUX into micromoles and vice versa.