Unfortunately our eyes are not at all good for measuring light intensity. We have to use a light meter, either a lux meter or a PAR meter to measure the intensity. If we use a light fixture than someone else has reported their light meter readings (versus distance from the light) we can make a good guess about what our intensity will be. But, I don't know if you are using a manufactured light fixture, that has some reported intensity measurements, or a DIY light fixture, or something else. So, I don't know how you can get a good guess at what your light intensity is.
You can buy a cheap luxmeter from one of several internet stores, and use that to measure the light intensity versus distance, with the light out in the air (cheap lux meters are not waterproof). There is little difference between measurements in air and in water. You can roughly convert your in air lux measurements to PAR by dividing the readings by 65 (or about that). If that says that, at the distance from your light to your substrate, you have 25-35 PAR, you have low light, or if you have 40-60 PAR, you have medium light. Above that is high light.