I was gonna get 250 health and put the rest in magicka. But I see now, that since I get 1-shot on Adept difficulty with 250 health, that's just not going to cut it. So... what is a good amount? And do get cute with me... funny people....

I am at 240 and I still die, not from one shots, but from poor fighting tactics.

Better battle management results in less health loss. If you see them setting up a power attack, back up or block it.

I have 330 after Enchantments and I got one shotted by a high end enemy.

You didnt say what level you are.

I just hit level 38.

250 health, 400+ magicka.

The difficulty in enemies is stupid. Either they are no challenge at all, or they kill me with one hit. There is no in between. I think the game has turned to rubbish and needs to be fixed. No enemies except the final boss should be that strong.

If you don't know what I mean, go to Lost Knife Cave at roughly the same level. You won't have any problems until you get to the room with the animal cages. Then you will die.

My lvl 24 character only has 100 health.

Use tactics and strategies to overcome any weaknesses.

I always give every character I make massive strengths and massive weaknesses, so I can develop strategies to utilize those strengths. I imagine the same might apply to the build you're using.

What kind of protection do you have?

It's just too bad that some enemies get bugged out damage and just one shot you even if you are blocking with a shield 300+ armor...

try cheesing your way trough
stagger em with firebolts so he can't even get a shot of
bring a friend and conjure an atronach to take the damage for you
all else fails, leave and come back at a higher level, some enemies don't scale at all (perhaps he's a level 50 or something)

it's what I've done so far and it seems to work just fine

You cannot overcome the one-shot archer kill. It's worse than the one-shot giant punt into outer space, in fact. Good luck to all of you who haven't run into this problem yet, is all I can say. If you keep playing Skyrim, you will run into it eventually, and then you will understand what I was talking about.

Let the companion tank.

My character is pretty powerful by now, but I can still get one-shotted now and then by elemental (fire, ice, shock) attacks, so I've taken to carrying restistance potions with me

perhaps he's just way more powerful than you are (soft cap at 50 so there is room)
not at level 38 yet (somewhere around 30, highest)
and during my playtrough I have come across things that one shotted me
when I went back later I could kill them (still hard, but more fair)
can't say for sure but perhaps this is the case with your encounter as well

one thing I do know
if something is constantly one shotting me I just remember it's location and get out of there
I don't immediately assume the game is rubbish and needs to be fixed because that would be quite silly