The formula in question seems to be:
DR = Ak/(1+Ak) where A = armor, k = 1/(50*moblevel)
So, for example, at level 5, with 100 armor, fighting level 5 mobs,
100*.004/(1+[100*.004]), or .4/1.4, or roughly 28%.
Fighting level 10 mobs, this changes to
100*.002/(1+[100*.004]), or .2/1.2, or roughly 16%.
Thus, when you gain a level, without increasing your armor, the character sheets displayed damage reduction shrinks slightly, since 'equal level mobs' are better able to penetrate your armor. Among other things, this means that there's no DR involved here, Armor just keeps getting better, the more of it you have.
Lots(!)
Molster over at Diablofans made a spreadsheet which details the amount of experience required to get Paragon levels with the new Paragon 2.0 system:
It is worth noting that these levels are in addition the 60 (70 in Reaper of Souls) character levels.
Best Answer
The level of the Greater Rift Keystone obtained by completing the Realm of Trials is determined by the wave you killed last. The wave level is determined by how fast you killed the previous wave (note the timer on each wave). It always starts at 1.
This metric has been slightly described in more detail for multiplayer groups. A group of 2 receives a key equal to the level complete minus 4, group of 3 gets a key equal to minus 5, and a group of 4 gets -6.
What determines how many levels Urushi gives to the next Greater Rift Keystone at the end of a Greater Rift ?
Of 15 minutes,
0-3 minutes taken: level + 9
3-4 : +8
4-5 : +7
5-6 : +6
6-7 : +5
7-8 : +4
8-9 : +3
9-10 : +2
10:30+ : +1