Diablo – Greater Rift Keystone drop rate and requirements

You are guaranteed a rare drop from all bosses on your first time killing each boss on normal difficulty. As of the 1.05 patch, you get "improved loot" for the first kill of a boss on every difficulty (its not clear exactly what this equates to yet). The intention isn't to allow players to farm, but to allow them to acquire enough gear to survive going forward.

After your first time on each boss, the only way to increase drop rate is through magic find and the Nephalem Valor buff

The Official Brady Games Diablo III guide says this about the loot system.

"If a monster drops loot, the quality of the item(s) dropped is most affected by the monster's level. If the loot bursts from an environmental object like a chest or barrel or pile of bones, the type/quality of items is most affected by where you're at in the game.

Other factors can come into play, as well. The fact is, Diablo III monsters can drop a wide range of loot items, but the odds of a low-level monster dropping a high-quality item are extremely small. So overall, it works just as you'd expect: The higher a monster's level, the better chance that it drops good loot after you slay the beast."

It also goes on to say;

"Again, there are ranges of quality for both the base item and for any affixed special properties. As we said, the primary variable that determines the quality of a slain monster's loot drop is the monster's level----the higher the monster level, the more the odds are weighted in favor of it dropping a higher quality base item, and the better chance that the game will also attach higher quality enhancements (or "affixes") to that base item.

These item/affix ranges are divided into "tiers" that correspond to the monster levels. If the monster is a low-level creature, chances are that any gear it drops will be selected from the lower tiers of items, and enhancements assigned (if any) will be from lower-tier affixes. Thus, each monster is said to have a "treasure tier" based on its level----the game tends to drop random items selected from item/affix tiers that correspond to the monster's level."

This is why the patch notes you quoted say "The new approximate drop rates are as follows" since drops from mobs are not based on act but monster level.

When I have some time I'll compile a list of the level ranges for each act/difficulty since the guide does have a complete list of the bestiary complete with monster levels.

To be honest I think you're over thinking it a bit. If you're level 60 then you should be farming the highest act you can reasonably do no lower than act 3 Hell mode and if you're lower than level 60 then farm the highest act you can farm efficiently. Farming one act back from whatever act is giving you trouble will give you a chance at gear you can use.

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• Act 1 has ilvl 61 mobs
• Act 2 has ilvl 62 mobs
• Acts 3 & 4 have ilvl 63 mobs

There are some higher (and lower in act 2) level mobs in both acts 1 & 2 but not many. Act 1 has one ilvl 63 (spiderlings) for instance.