advice on my small scale grow

[Volker]

would it be better to add another plant in there and use all the screen?

That really depends on the strain. With something like Lemon Skunk - or other short, squat indica - you could easilly do 4 plants with that screen and 250W of flouro. If you have a sativa dom, you might barely do 1 by LSTing the hell out of it.