Dude as far as i know there is no direct relation between height and yield, it all depends on the strain and the conditions u grow ur plants that is going to determine the final result yield and quality wise...there is no optimum height for flowering, u can flower ur plants any time u want, what u should be aiming its not height but the plants age, imo u should veg the plant long enough as it would in veg in outdoor conditions so aim to at least 4 weeks in veg....plants that are flowered at 1 foot tall can reach 4-5 feet in flower and plants that are veged 2 feet tall might only get at 3 feet after flower so once again there is no direct relation between these to cycles....so i´ve read.