How to use Lagrange Interpolator?

I have previously tried Linear and Spline Interpolation for my Wavetable synth, but I now I want to give Lagrange a go and compare the difference. I have seen that a Lagrange class exists in JUCE but I am unsure of exactly what to feed into it.
More specifically, if I need to generate a sample from index 500.576 of a 1024 sample wavetable, would my input to the Lagrange be samples 499, 500, 501, and 502?
Also, can somebody give some advice on an ideal “speed ratio” for this kind of application? That is, is four input samples per one output sample likely to produce industry standard results?

The LagrangeInterpolator class is for resampling a stream of data, rather than interpolating a signal to specific points between your input samples. I think this is where your confusion is coming from.