What software are you using to process your images? If it is Pixinsight my guide covers most of the issues.
I usually use 25,000 ADU for my flats. I use a flat panel with a number of sheets of white paper to reduce the brightness to achieve an exposure time of around 3-4 sec. I also use a flat dark series taken at the same exposure as the flats to calibrate the flat (again this is in the guide). As the flats are always taken on the same session as the lights I never change the gain or offset (120 and 30 respectively). The white balance should be left at 50 for each setting in the indi gui panel.
The longer exposure is needed as the sensor response is far from linear at very short exposures.
You say that your flats have a red cast. This is a bit confusing as flats should not have any colour as you do not debayer the image. You just do a simple integration to create the master flat but do not do any other processing. If you view the resulting masterflat after stretching it should be easy to see variations in uniformity which relate to dust and errors in the light path which your processing will calibrate out of the light frames.
All the calibration of light frames using flats and darks are done before debayering the lights. Debayering is not done on flats or darks.
Let us know if this helps and let us know what processing software you are using. I only have experience with Pixinsight but there are other here that use other processing software.