The values R1=50 and R2=12 are for the π crossover, which uses a bypass capacitor across R2. R1 is the "load" for the high-pass filter and R2 is a series attenuator. Capacitor C1 is placed across R2, and is 0.22uF for a 1.6kHz crossover and 0.66uF for an 800Hz crossover.I say all this as a reminder that the three components are part of the filter formed also by the compression horn and the Butterworth crossover filter. The thing is designed to provide specific damping at the crossover frequency, so that the first couple of octaves are flat before augmentation starts. This is all outlined in detail in the π crossover document.
If you wanted just a flat attenuator with no compensation for the compression horn, you would choose R1 and R2 values that added to be approximately 8 ohms, but that acted as a voltage divider, cutting power to just under 2%.