What I recommend is taking the default LUT used by unreal (you can find it in the engine files) and export it. Then take a screenshot of your scene. Import each of these, and then duplicate your LUT layer (so you now have one screenshot, and two of the same LUTs), then merge one of the LUT layers with your image. You should now be able to hide your backup lut, and tweak the screenshot layer until you get your desired results. You can then extract the LUT from your screenshot layer and it will be exactly what you are looking for.