Gems

I read that mastery gems are best for paladin tanks but ask mr robot keeps telling me all the time to use stamina gems. Do you guys know why? I hear ask mr robot is pretty reliable.

http://us.battle.net/wow/en/character/stormrage/Arkknight/simple

With your gear level, for normal mode you don't need any more Stamina, in my opinion.

For heroic mode, I would still gem Mastery, but you should ask your healers if they feel you need more buffer with your hit points.

I have no experience of mythic difficulty.

AMR doesn't have the concept of dynamic stat weights. It uses a fixed set and assumes those are constant regardless of changes in gear, talents, etc. For most stats, this is a reasonable approximation, but it's generally a poor approximation for Stamina. Stamina is very valuable when you're undergeared, but loses value as you get more of it.

The stat weights AMR uses by default are ones I generated with an undergeared character, leading to an over-estimate of stamina's value. If you feel comfortable with the amount of stamina you have, you can customize the stat weights and reduce the stamina value.

Health is always a weird stat to measure quantitatively.

You really only need "Enough".

If you never drop below 50% hp, you technically have "too much" health since 50% of your hp is never being utilized. In a perfect world, every hit would take you to 1hp and then you would be healed back up again but that is unrealistic.

Also you have to consider the healing throughput that 1, 2, or 3 healers can put on you. If you, hypothetically, get torn apart but have a ton of health, your healers may struggle to have the HPS to ever catch back up.