If you just want to top the plant you really cant go wrong, I always count up to the 5th true node and cut there, however if I wanted to keep that cutting to make a clone I wait till the plant is 8 or 9 nodes tall, then just cut off at the 5th. The cut tip I then manicure into a nice piece for propagation. If you are REAAALLLLY not sure, take a picture open it in paint and draw and arrow on the plant where you were intending on cutting. If you posted that picture it would be really easy to tell if you have the right idea, The first plant I ever topped was on accident.. hope this helps man.