Saving and restoring backlight level across restarts
AppleBacklight.kext stores the current brightness level in NVRAM for restoration across a reboot. It stores it in NVRAM variable 'backlight-level'.
In order for save/restore to work, you must have properly implemented NVRAM. For some laptops native NVRAM does not work, so you need EmuVariableUefi-64.efi. Without EmuVariableUefi-64.efi in drivers64UEFI, native NVRAM is used. With it, emulated NVRAM is used, saved to disk (nvram.plist) at shutdown, and loaded at startup. In order for emulated NVRAM to work, you must have installed "RC scripts" within the Clover installer. Failure to install "RC scripts", but having EmuVariableUefi-64.efi present in drivers64UEFI will cause NVRAM settings to never be saved in nvram.plist.
Also, make sure config.plist/SystemParameters/BacklightLevel is NOT present in your config.plist. You don't want Clover setting the backlight-level to something other than the NVRAM value.
Keep in mind that if you visit Clover Options -> Graphics Injector, that the default for Backlight Level will likely show as 0xFFFF. If you return from there, Clover *will* inject that value, overriding the NVRAM, which is probably not what you expect. To avoid that problem, always blank the setting before exiting from the Options -> Graphics Injector screen.