Here is how I would do it
1- Reorder your image to llll so you have luminance information
2- Analyse the desired range on a flat portion of the image (blur a region if needed)
3- Once analysed, get rid of the reorder node
4- Plug a brightness node to your shot
5- Check your shoot for the desired brightness. If, let say, your desired exposure is at frame 44 , check what the Analyser got at that frame in the area1AverageRed box. For this exemple, lets assume the result was .144
6- plug that expression is the brightness node;
.144/(PixelAnalyzer1.area1AverageRed)
This will work for luminance. IF there is a color shift, skip step 1 put a mult instead. Then put the result in each channel. Not only it wil lfix the lumincance, but the color difference too
Tell me if it worked !