I wish I could give you the right portions but I don't know myself. I buy the pre-cut slaw mix from the grocery store. I use mayo, buttermilk, salt, pepper and celery seed. My guess is the it 2:1 mayo to buttermilk. I start with 2 giant dollops of mayo with a serving spoon, then add buttermilk to thin it to the right consistency, then the rest of the seasonings to taste.
If you've ever had cole slaw from Zestos in Columbia, its a real close clone.