how much do you suggest cooking down the liquid in step 3?

Until the recipe author replies, perhaps you could wing it: try reducing by ~ 1/4 to 1/3. Would give you about a cup of liquid more or less. Adding the cornstarch and simmering briefly to thicken will reduce it further. You're adding a good amount of solids (1 lb chicken plus the broccoli). Depends also on how "saucy" you like things.