around 12" would be better but as you know your yield will not be that great but with the limited height you have. If you can get a product called Bush Master from Humbolt County you would do better. This product is made for growing conditions like yours. It will stop vertical growth so you could grow your plants taller and have more yield in the end.

http://www.bghydro.com/BGH/Itemdesc....EGBUM08&eq=&Tp=

Check it out yourself.