Ok well ive got a bit of a rule of thumb for you then since i am a Sion main.
Lane Sion
He gets 2 stacks for small minions and monsters and 10 for everything else, Since you get small minions more often i take that number and add 1 to compensate for the occasional big monster/cannon minion kill and assisst.
Sions cs x 3 = bonus health stacks
100 cs = 300 hp
200 cs = 600 hp
300 cs = 900 hp
This isnt always 100% correct but on average this is very very close.
**Jungle Sion
**
In the jungle you get less cs total but there is a big creep every camp. just double the multiplier and you come very close
Sions cs x 6 = bonus health stacks
50 cs = 300 hp and so on
You can actually see his stacks but a quick look at the score board reveals his cs too and you can use this to get an approximate number without having to look it up specifically