I take a combination of 75% green channel and 25% red channel on my pyrocat negatives (stained negatives) in the channel mixer from my 48bit rgb scans. I have an action that opens up the scanner software, I do the scanner driver stuff and hit scan, and once it's in photoshop the action automatically does the channel mixer, converts to grayscale, opens the Save As... dialog box so I can choose where to save my image, and then it closes the file.

I use 75/25 because that seems to give me the best combination of tones and minimal noise for MY workflow. Epson 10000XL scanning 8x 4x5 negatives at a time at 2000dpi which will eventually be printed to 16" x 20" @ 360dpi.