Do you know where the cervix is? Have you felt your own? If you can't palpate the cervix, it must have been at 10cm dilated... We practice on dummies at my university. You stick your fingers in, and use your brain to think about what you're feeling. If the cervix emits a fingertip, it's 1cm, if it's more, you need go with your best estimate. Can't fit a ruler up there see. You need to practice, that's all. Have a professional do the vaginal exam as well and compare.