If you're new to tanking, a good health pool is going to give you more leeway to learn encounters. Later when you're more comfortable with mechanics you can star to dial back on health. Basically it's your room for error.
I'd say for most anything, on your average player, 37.5k-40K is a decent number to shoot for. You'll want to be able to get closer to 50k for certain specific things, but you can also go as low as 34-35k and be fine if you know what to expect.
Keep in mind your damage shields (igneous, absorb missile) scale off max health, and green dragon blood heals for 33% of your health so more health makes those skills stronger.