c# - Calculate a pixel value

Question

If I have this code in C#:

int index = 10;
float value = 20;

float* pixels = getPixelData();

pixels[index+1] = (byte)(value/256);
pixels[index+0] = (byte)(value & 0xFF);

Then, the question is how to calculate the 'value' if I have:

int index = 10;

pixels[index+1] = X;
pixels[index+0] = Y;

float value = ?