This is a demo tutorial. While there is no step-by-step commentary available (yet), the live demo below is fully functional. Feel free to open it for full source code.
Normally, Funnel chart calculates and shows percentage of each slice based on its value in comparison to the sum of all values. This demo shows how we can use adapters to calculate percentage based on the first slice value, rather than the sum.
See the Pen amCharts 4: Funnel chart with relative percent labels by amCharts team (@amcharts) on CodePen.24419