BE aware there are two reservoirs on the ABS unit!
If you try to flush the brakes from Handlebar lever or foot brake to the wheels you will possibly empty the reservoirs and wreck the ABS
And also If you pads are half worn as long as you have fluid in the glass ??? It will be fine If you top the reservoir up and fit pads after there won;t be enough room for the brake fluid pushed back from the callipers
Maybe, you should step away from working on your bike. If you need to ask something as basic as brake fluid level, then messing about with your bikes brakes is likely to not end well.