Usually impulse waves stay within a channel, but sometimes wave 3 can be strong AND wave 4 can also move very sideways exiting from the channel you've created. In order to project the end of wave 5 in such cases try connecting waves (2) and (4) and draw a parallel line intersecting wave (1); that will project the end of blue wave (5) as you can see. It's not very well known technique.
Note: follow the blue labels
Note: follow the blue labels