Welcome back! This is a really great question, thank you for writing about this. I'm sure you will help others here as well! You are spot on with keeping your head down with arm and ab work on the reformer. What I would recommend for crunching work would be keeping the head flat, and either place your hands under your tailbone in a diamond shape (with no handles), or keep the handles in your hands and reach them up toward the ceiling with your finger tips making a diamond shape. Press your hands forward into the tension about 3-4 inches, so you keep the tension reaching forward and not scrunching back toward your neck. That will activate your transverse abs, or the deepest layer. Your legs should be reaching up toward the ceiling, heels together and toes apart with a little bend in the knees. From there, lower your legs down a few inches, deepen your abs and lift them right back up. This is effectively doing a crunch, but simply from the lower half instead of the upper half, and it is very effective! Since your neck is flat though, you don't want that range to be very big with the legs. If you go too low, it could pull you into an arch in your back or neck, and we don't want that to happen. ;) I hope that helped and wasn't too complicated of an answer!