Lamp to plant distance: is 40cm enough during flowering phase?

Unfort I cant give you a set distance and be confident its the right distance as every strain is a bit diff. Even diff phenos of the same strain can be diff. One plant might love 40cms while another gets light burn. The best advice i can give you is to use the charts the light company gives you as a guide and tweak it as needed. If the company says 40 cms for example during bloom, Then id go to 50 cms for 3 or 4 days and see how plants look-if good then id drop to 45cms for a few more days and see how plants look, if good then either leave it or drop it to 40 cms and see how they go. if at any point they start to look less than happy, you know that distance is too close and to raise it back up to the last distance the plant was happy at. Thats if your trying to squeeze every last watt and photon out of your light and push the plants as hard as possible. This is an added cause of stress, some plants can take the added stress of very bright light just fine, while others will very quickly show you they dont like it. If your not trying to push the plants as hard as possible, then just find a distance thats working just fine and run with it. if 40cms is working then stick with it, you could drop down to 30cms in theory and most plants would be just fine, ive heard of pple going as low as 8 inches or around 20 cms but Im assuming thats with a 100W light, So if 40cms is working and plants look happy, then run with it. If you want to try going lower, drop it by 5cms each time and give it 3-4 days to watch and see how plants respond before deciding if you want to go back up, leave it or go lower again.