Instead of using low-fat buttermilk (which you can never overcome the runny problem) I splurged on full-fat. As for low-fat mayo, a lot of low-fat mayos are garbage and just have more soybean oil instead of the eggs, and the eggs are what give it flavor. Soybean oil isn't good for ya either.
I've found the best lowfat mayo after literally trying out a dozen kinds: Lemonaise Light. It's pricey, but gives a great mouthfeel. It's available on Amazon if it's not in your local grocery store. Also, I used much more buttermilk than lemonaise, because in the called for amounts I got a very thick texture because both full fat buttermilk and lemonaise light are on the thick side. I also used chives as my fresh herb. Delicious!