First, realize that the Achilles is the tendon that attaches BOTH the soleus and the gastroc to the heel. When you stretch with your knee bent, you have disengaged the gastroc (which crosses the knee) and are stretching only the soleus (which doesn't cross the knee). When you stretch with your knee straight, you are stretching both the soleus and the gastroc, but primarily the gastroc. The pro stretch tool is good, but honestly you can do essentially the same thing by putting a 2 x 4 longwise on a set of stairs and putting your foot on it at different heights and leaning forward. Another good stretch is to lay flat on your back, take towel and loop it across the bottom of your foot, keep your leg straight, and pull on the towel to stretch your calf. The drawback here is that you have to pull pretty hard to get a good stretch. The good thing about it is that you are able to relax your leg for a better stretch compared to when you are standing. Hold each rep for 30 seconds. Holding it longer doesn't give additional benefit. Repeat at least 3x per session and try to do it 3 different times during the day. You gain flexibility by stretching often, not be stretching a lot at one time. Hope that helps.