GrowCNanswered grow question 4 years ago I know this isn't the simple 50cm/50in answer you are looking for but... I downloaded a lux meter app for my phone. Then you can measure the light at the top of your plants and convert that number to PPFD. Once you know hot to do that it's easy to position your light for each stage of growth. It also allows you to adjust the light's output in addition to it's height.
I use HLC260 white LED lights. The conversion factor for my lights is 65. So I measure the light at the top of my plants and get 40'000 lux. I divide 40'000 by 65 and get 615. Then on the PPFD chart (https://www.ledtonic.com/blogs/guides/dli-daily-light-integral-chart-understand-your-plants-ppfd-photoperiod-requirements) you can see what light level you need in relation to how long you're running the light.