you can flower any time you want.... usually people will wait until they have at least 3 nodes.. another thing... you want to flower when your plant is thriving... when it is strong.. if you are suspect of a nut deficiency.. or rootbound you should not flower.. you plant is plenty big enough to flower...

since you only have one plant you may want to wait....
then again you only have a 400watt bulb which means it won't penetrate deep into the foilage so you should flower now..

But you better clone first.. : )

good luck