High Braya,
The height of the first netting will be higher or lower, depending on the variety of cannabis being grown. Around 20 cm above the top of the pot is the right height for Indica varieties, 30 cm for Hybrid and for Sativa would be the best height around 40 cm.
For the second net I would recommend to add 50% from the hight that you choose for your first netting.
Then your top colas will grow perfect into your second net. And will use this net the best for creating a screen of green and support.
Happy Growing! 😋