If it was me in your situation, the thing you need is a height to work to, otherwise you could be pouring slc forever. If you can get a bag of plastic packers, of varying thicknesses you will be able to get it somewhat nearer than just pouring and hoping. If you find the high point on the existing poured slc and then work from there.
What you need to do is silicone the first packer (cut them into 3 pieces) to the high point. Sit your level on top of the packer and approx 600mm away from there use more packers until its level. Move round the room uniformly fixing the packers with silicone. Once set you can then pour the new slc until it's level with the top of the packers
This is what I mean, unless you've already done it