I Have A General Prescription For 1mg Lorazepam As Needed.

I have a general prescription for 1mg lorazepam as needed. I get 10 pills about every two months or so, to help with anxiety of medical tests, appointments and treatments. I'm a very large lady. 270 lbs. I am also claustrophobic. I have a 1 hour full spine MRI booked soon. I don't generally feel any result noticeable from lorazepam, but I assume it helps. How much would be recommended for my MRI appointment and how soon before my test should I take it?
Some background, I have chronic pain due to ossification of my posterior longitudinal ligament and a bunch of other things going on. As such, I am quite opiate tolerant.

Thank you for your time and attention and opinion.