The simple answer is: you don't.
Personally I use a combination of weight and waist measurements to monitor my progress. I don't know the exact body fat percentage, but if the waistline is going down, I know I'm probably progressing, even if the weight isn't moving too much. You could also measure other body parts (i.e. chest, thighs, arms), depending on where your body prefers to store excess fat (this varies between individuals).
I've also done a test similar to the one provided by your scale at a fitness center, but even if that is also inaccurate (although assumed more accurate than a (cheaper) home device), the absolute measurements are not really all that relevant. What is relevant is comparing measurements over time.
Say, for example, that you measure your BF at 30% and then, six months later, you measure it again at 25%. You don't know if you're actually 25%, since the measurement is inaccurate, but you know you have less BF now than the last measurement.