[Addos (Adam)]

The 1/3 peak histogram levelling is guidance for DSLRs.

As others have said, for dedicated astro cameras you want around 50% available. IMX571's have 65k max well depth at unity gain so ~30-32000. Advice that flats should be 1 second in length is bunk - you use whatever exposure time gets you to that adu level (special cases like imx294's notwithstanding).

Flat panel / light source brightness is then nothing more than a variable in that calculation. Dimmable drawing/led panels usually have 3 or 4 brightness settings, so between the different filters you should be able to find something that works. use nina's flat wizard/dynamic flat mode and it will do it all for you, hunt down the optimum exposure time to reach set adu range. If it (or you manually) cant hit a short enough sub length to hit target adu (common with the lum filter), damp the light source with some plain white tshirts / other fine woven cloth.

Gain/offset settings and frame orientation if changed may impact the quality of the flat subtraction, so try to keep that consistent. not super critical however there are post procedures that can reasonably mitigate alot of flat correction related issues, albeit time consuming and fiddly. you also dont necessarily need new flat sets each time you shoot, although with a newt you'd have to do it far more often than I do with a refractor (larger, open optical train gets dirtier easier and more often, collimation shifts).
Best of luck!