Best Grayscale method for checking contrast?

While designing art assets in Photoshop, I've always tried to use the Grayscale method for checking the contrast levels. Unfortunately, different articles/sources recommend different methods of desaturating an image, and they all produce vastly different results:

A) Create new layer, fill in #000000 Black, set blending mode to saturation

B) Hue/saturation adjustment layer, saturation slider down

C) Vibrancy adjustment layer, saturation slider down

D) Switching to a Black and white gamut proof

E) Black and white adjustment layer

Which one do you use (if any) when designing your artwork, and does anyone happen to know which method produces the most accurate result for checking contrast? Cheers!