Spell powerFollow

Ok first off i'm not raiding, i'm leveling so take that into account when answering. How much does spell power really add to locks as they level. The reason i ask is that at lvl 40 i'm starting to get items that have spell power on them but don't have as much in base stats. Do they take and add the spell power as a percentage per tick on our curses and does the spell power just add direcly to our shadow bolts. IE 50 spellpower 50 more damage to our shadow bolt and 50 more damage over the full curse? Any help to clarify how this actually gets added would be helpful.