Optifast 70 is 80 calories per packet for a total of 400 calories per day, while the 800 is 160 calories per packet for 800 calories per day.
I have used 800 on my own to help with maintenance, and I can say that weight loss on 800 is pretty similar to that of 70. The major difference is taste. The 70 tastes awful (especially now that orange is discontinued), while the 800 is palatable.
I hadn't heard of hmr, and looking at their website I don't see a liquid only option. Are you on liquids only, or is there food involved?
Good luck with your journey!