It's much easier than you think. When you're walking, just note number of current steps at two points and then see the distance in any map service. For example, your route from bus station to home. Write number of steps at bus station (let's say 3451) and when you entered your home (3921). You passed 3921 - 3451 = 470 steps. Then see the actual distance in Google maps. Let's say it shows 300 meters. So your step length (in settings) used in app should be 300 / 470 = 0.64. After you have set it, the displayed distance at the main screen should be more accurate.
