If you want a strong multi stalked plant, topping must be done early. Wait 'till your plants get thier first set of 5 bladed leaves, then nip odd the tip, leaving a minimum of two developed nodes behind. These two nodes each will grow 2 branches, resulting in a 4 branched plant. Using stakes, carefully train these 4 brances outwards from the centre stalk. Be mindful not to break off any branches. You can leave this to grow as is, or you can repeat the process again to make an even shorter, bushier plant. By training large brances to grow as far apart as possible from each other, you open up the structure of the plant and allow more light exposure to the lower nodes, thus increasing yeild. The bottom line is to train them to fill all availiable space. Farmer Fred's plant is a perfect example of this.
With 400w, you will get about 1 1/2 - 2 feet of vertical budding area on each plant (if that makes sense). Measuring from the top of a plant that has already stretched and is well into bud (say 3 weeks in) , cut off all the small scrawny braches and leaves below 2 feet. This lets the plant concentate on the good stuff up top.
Depending on if your bulb is horizontal or vertical you want it about 18" away from the tips with horizontal and around 12 with vertical.