With my RASA8 Iv'e shot 184 light subs of a galaxy field under the new moon, with som pretty heavy gradients caused by the moon. 

My normal flats wont work for this as it only takes care of vignetting. So I made a synthetic master flat from the stacked lights in Pixinsight, a master flat that to me looks very promising. (I first tried to make a synthetic flat in APP, but due to the limited gradient options it didn't turn out good) This PI master flat was then saved as a 32 bit float fits.

The problem is, and I've tried everthing solution I possibly can find on this, but no matter what I do, APP does not calibrate the lights with this master flat. The results look identical with and without this master flat loaded.

I am able to fix the gradients with the Light Polution Tool, but I would reaaly like to get the synthetic master flat thing working as I see a lot of potential in this for different reasons.

I've attached a light sub and the master flat I made in PI.