Whats 40cm in inches :P ok google says about 15" That's decent. It's possible to go 12" sometimes but thats dependant on diode count and intensity. Not all lights are created equal. 40cm is good though, you'll be ok. Just keep watch cuz the plants will grow up to it and if you don't monitor regularly, you'll walk in one days and your plants are 6" from the light and are blotched with white from light damage.