High TokeGoat,
You can start at 50 cm and down this every other day with 5 cm till you notice the beginning of heat stress (leaves take the form of a taco).
From there you lift it back up to the distance you had before your last lowering and this should be your perfect distace!
You can also test it yourself. Put your hand under the lamp for 1 minute on the distance that you want to put your plants. When it becomes warm and feels unpleasant it is too close for your plants aswell. When you do not feel any heat from your lamp your plants will be fine!
Happy Growing! 😋