I've come to the conclusion that its best to top from 5-6 nodes down to 3rd node. In your case, since it's an autoflower, it's not advised since
1. The part of the plant that you chopped off will not restart grow cycle.
2. Topping is High stress which is said to waste a week of the autoflowers life in recovering instead of growing further.
Hope in your case you manage to work it out and have a good yield. Good luck with the rest of your grow