Im in my 3rd grow.
I veg for about 1 and a half month,i top every girl to achieve more tops,my temps are 82F,i use All Mix soil by biobizz, with organic nutrients like Bio Bloom, Bio Grow,Top Max,Bio Heaven...
I put my plants in flower when they have 12 inches-15 inches.
I got a fan circulating the air inside the growroom and a 107 m3/h intake and outake 190 m3/h.
400w hps with an adjust a wings reflector.

That sounds like a great set-up. I've used Biobizz before, good results.

15 gm a plant is not good. I've done a lot of grows with 400 watt. Over 400 grams is normal. Average number of plants = nine. So an average of 50 gm/plant. I use 3 gallon plastic grow bags with rich organic medium, home mixed. pH of well water 7.2-7.9. Water is drawn into a 5 gallon tote (at this time of year the temp is low 50ºs), Correct pH to 6.2-6.5. Run airstones for 24 hours. Temp goes up to 65º - 68º. Check pH, water the plants.
I do veg for eight weeks or even more. But with 5' height six weeks sounds good for you. I crop at the third and six nodes. Like when they are tiny. I trim all the lower growth but I don't worry about height. LST would word great for you too.