Tank emptying harmful?

Hello! I’m a bit unsure, so my question. I’ve been driving a used BMW 116i, 2l engine, 12/09. The new one has a digital reserve display and asks me to fill up at 100 km residual range. How bad is it now, if I fill up much later – e.g. at 20 km residual range or even later? Thanks and greetings, RI