Another measuring stick that we use is to always keep both hands within our peripheral vision. Even when it is appropriate or necessary for an arm to not be centered in front of your torso, if you keep the hand within your peripheral vision it creates a more stable posture as well as giving your brain a greater amount of information about your posture to use in learning to coordinate your body. Of sourse there will be specific instances where you have to get an arm behind you to be able to do a technique, but this guideline applies to the vast majority of aikido.
In order to determine where your peripheral vision ends, look straight ahead and stretch your arms out to either side and slowly bring them horizontally to the front while wiggling your fingers. You will come to a point where you can see your fingers when they wiggle but not when they are still. This is the limit of your peripheral vision. Now, move your hands up and down from this point, locating all the edges of your vision. Your arms will end up describing a cone of about 150 degrees in front of your body. If you always keep your hands within this cone then you will be more stable and learn aikido faster.