Colder weather means full-strength nutrients (20ml per gallon of A and the same for B). I switch to full-strength when the temperature no longer reaches above 85 degrees. This applies whether the plants growing in your Tower are all mature or a combination of seedlings and mature plants.
Use half strength nutrients (10ml per gallon of A and the same for B) in the summer when the temperature is steadily reaching 90 degrees or anytime of the year when your Tower contains all new seedlings.
The graphic above provides an easy nutrient reference guide. Remember, nutrients should be added each time you add water to the reservoir. See page 3 of this document entitled Water for a handy way to measure the water in reservoir.