The key point to grasp is the role of sumti and selbri. I see this is no problem for you, you got them right.
That said using {ku} to explicitly terminate the sumti or {cu} to separate the sumti from selbri is a matter of taste.
{lo merko cu dunda lo plise} and {lo merko ku dunda lo plise} mean exactly the same.
Using {cu} is the way thought by CLL and L4B. Most of us learned Lojban that way.
The use of {ku} is what the authors of wave lessons have thought on #ckule for quite some time.
Personally I don't really mind, I just try to stay consistent and use one or the other without mixing them too much.