getPointAtIndexInterpolated float value

I am using getPointAtIndexInterpolated to draw a circle moving in a path. I created the polyline like this
ofPolyline poly;
ofPoint p1(400, 400);
ofPoint p2(500, 300);
ofPoint p3(600, 600);
What would be the float value to get the start and end of my line?

getPointAtIndexInterpolated i similar to getting a point by it’s position in the polyline but you can pass a float number instead of an integer to get an interpolated point, so if your polyline has size = 200 the float number will go from 0 to 199