I happened upon a very good way to check the progress of your Ab Rehab program. I do not recommend trying this if you are not currently on the Ab Rehab Program!
Run a bath and recline in the tub with your head resting on the back- I have a soaker tub with a gentle recline so this is comfortable. Allow your upper body to float and your shoulder blades will gently lift away from the tub. This will passively engage your abdominals. Now perform the self check in all 3 spots. If your connective tissue is strong (one of the first signs of progress with the program) you should be able to feel the separation between your rectus abdominus fairly easily. Then gently sink your body so your shoulder blades are resting on the tub again and you will feel the separation open as the rectus relax. You should add a fingerwidth to the floating measurement, as the abs are engaged- that's what it worked out to for me, although since I haven't checked anyone else this way it could vary from person to person. I would appreciate any feedback if you try it!