If you have more than one sintering oven it is very important to keep them calibrated the same. Use a process temperature control ring (PTCR) from Ferro.com to achieve the most accurate calibration temperature. B&D Dental Technology (Origin) also sells them. You place a PTCR in with your zirconia crowns when sintering. After it cools measure the ring. The more energy it absorbs the more it shrinks so you can accurately gauge the temperature inside your sintering tray, whether it is a short or long cycle. If your ovens are even 10 degrees different you will get about a quarter shade lighter in the hotter oven. With this in mind if all your shades are a little dark you can try bumping up your temperature 10 degrees or vice a versa.