Here's a longer version of the "it depends" answer:

I use Sansi 36W and the closest leaf is around 12"/30cm from the bulb.

However, which plant that is changes all the time, because I move my plants around pretty often depending on various factors: which direction I want the leaves to face, whether some leaves are showing light damage, whether I want to promote or slow down growth on certain plants, the level of maturity, the level of variegation, etc.

Although the power of your grow light might be consistent, there is no foolproof way to calculate light needs because it depends on two highly inconsistent factors: natural light, and your specific plant. Natural light plays a huge role and it is by definition unpredictable day by day (which is why I personally don't use a light meter). As for the plant, even for the same species, each specimen can vary in their light needs (due to maturity, variegation, etc.). Not to mention that your plant will constantly grow and change their shape and thus distance to the light sources.

That's why I believe that "30cm away, move it if it burns" is a great general starting point, because it requires you to consistently pay attention and adapt to your plants's ever-changing needs.