How do I figure out hanging height?

Had a little issue with finding the right height last grow and saw a thread talking about par meters and lux meters but I’m not sure exactly what I’m looking at when it comes to buying one. The thread I saw says that lux meters can’t be used for blurples to measure the hanging height. What gadget can I buy to figure it out?

Start with the manufacturer spec sheet that came with the light. Should have heights for veg and flower.

Lights like everything else in growing is variable. Going to depend on your plants and environment. Good luck.