Not necessary to turn over. What I do is place the ruler against a wall and set the tortoise on it against the wall and you only need to read the measurement on the back side knowing the front is in the right position for a correct measurement.
Measuring tape works. Plus the torts lungs are located on their back. That's why when they flip over they poop to relieve pressure if they can't upright themselves with their neck. Not that a few seconds would hurt, but it's uncomfortable.Thank you I saw a video of someone turning there tort over to measure the underside of them