hey man... i have read allot of posts about scrog and allot of peoples grow journals.. and it seems that the best way is to flower when they are 8 to 12 inches tall.. i would put the net like 11 inches up and flower when tho tops come up about an inch above the net... then you start training it... the plant will go into a growth spurt cause it thinks its too short to flower... that spurt is enough to fill in your net ... especially if you were topping and designing your plant in the veg state in order to produce more branches... check out my grow and relax.. you'll do fine... people that do longer vegging under a scrog end up with more salad than buds...
good luck with your grow.