You can select three things here. The best will be if you leave it at default settings.
25 indicates overbought.
75 indicates oversold.
But, as it has the impact from the of the stock apart from prices, it can not be named as a lagging nor a leading indicator.
Use it for divergence testing and as an additional method for confirming Potential Reverse Zones (PRZs)
If you want to use this in a black chart, use this indicator which shows the lines in white -