I take it you have 13 orcs (78pts/6) and you have about 39pts left over...till 600.
Then that would give you 30 models foe 600pts.
I´d say that since you took some elite trooops in already then you should try to get as many cheap orcs as possible.
I don´t know exactly what The Knight of Umbar does but I do have some thoughts and as one of the Nazgyl I would keep him.
The only thing which I would change is Remove the Troll Drummer, that would give you with left over points aprox. 189 pts, and then i would get 17 orcs and then you woul have 87 pts, left which I would spend to get Black-Uruk Guard Captain.
In the end you would have aprox. 45-55 models.