The difference between heroics and raids is quite simple: heroic bosses are level 82, while raid bosses are skull (which is counted as lvl83 when calculating combat tables).
From there, you can get everything other bit of info you need from the Wowwiki article on spell hit, which is quite informative.
To summarize the important points:
- You have 94% chance to hit a target 2 levels above you with spells
- Therefore, with 0 hit and no hit talents you have 6% miss chance against heroic bosses
- As an Elemental Shaman, if you have 3/3 Elemental Precision you are therefore short 3% hit
- To make up for that, you therefore need 79 hit rating on your gear.
- Note that if you're a Draenei, your racial makes up for 1% of the hit you're missing: as such you will only need 53 hit rating from gear to make up the remaining 2%.