@ShinyHero has it. And like
@Goesid said, here comes my longer explanation lol.
So as mentioned, each average equipped ilvl above the bracket ilvl (24) increases your stats by 0.1%. So 29 is 0.5% and 34 is 1% and 41 is 1.7%. It actually also includes decimals, so an average equipped ilvl of 41.36 actually gives you 1.736%.
The breakpoint comes in on the stat side. You don't have half an agility point or half a stam. Those have to be whole numbers. Say you have 50 agility. If you get to ilvl 34, you get 1% extra stats, which means you'd get and additional .5 agility so you would be at 50.5 agility...but since you can have only whole numbers, it still stays at 50 agility. At ilvl 43, you've got 1.9% stats and you'd be at 50.95, which is still 50. So you need to actually reach ilvl 44 to get 2% increase to get to 51.
Your base stats come from your class/spec template and (probably) your race. So the actual breakpoints depend on which class/spec/race you are and blizzard tweaks them every so often to balance high level play. So its rather hard to reliably have breakpoints to go for. In general, just go for the highest ilvl you can, but don't stress since its like 1 or 2%.
More importantly, make sure you've got something in each slot, use the correct gear (pallies use plate, not mail/leather/cloth), use a wand if you can, use a shield over an offhand, and use the proper weapon category. All of those carry much more weight than item level.