A fan created some useful interactive tables that let you figure exactly your upgrade odds for any level Legendary Gem. Just locate the level of your LGem and you can see the exact odds for each of the three rolls, and the cumulative odds to get 1, 2, or 3 upgrades.
The calculation for LGem upgrading is actually very simple, as I think most players know. Above you see a screenshot of the little table on the DiabloWiki.net Legendary Gems page, which I thought was sufficient.
The upgrade odds are pretty generous, with 60% or higher so long as your LGem is at least equal to the level of the Grift. If you want to be guaranteed of success with all 3 rolls, you need to clear a Grift 12 levels higher than your gem. So if you’re working up a brand new gem and don’t want to waste time clearing higher Grifts than necessary, but still want 100% for all 3 chances each time, you’d go 12, 15, 18, 21, 24, etc.
Here are some screens of the three different options on the automated charts, showing 1-25, 10-35, or 1-50, and check out the source page for these all interactive.
Note that the odds are sort of oddly expressed for rolls where the odds are above 60%, but decreasing with each success. For instance, if you start off +10, your rolls (if successful) would go 100%, 90%, 80%. The chart shows this as: “Chance of 2 upgrades: 27%. Chance of 3 upgrades: 72%” I had to look at that a minute to figure why the odds were worse for 2 than 3, before I realized it means “exactly 2,” not “at least 2.” So yes, you’ve got lower odds to get exactly 2 successes than 3 successes in this instance, but your odds to get at least 2 out of the 3 would be well above the 72% chance to get all 3.
Do you guys have any interesting strategies for upgrading LGems?
I’m dividing my play time between US and EU, and I’m HC so I’ve lost my highest level gems a couple of times. And I’ve compounded that by experimenting a lot with different gems, so I’ve leveled multiple gems up to level 10 or 12, rather than sticking to 2 or 3 and getting them up as high as they’ll go. Note that this is neither a wise nor an effective strategy, but like I said, I’ve been enjoying experimenting.
Hopefully other players have done it a little more scientifically, and I think there could be some interesting debate on the best technique. Would you rather level one gem to 25 as quickly as possible, then leverage its upgraded power for faster/higher clears to zoom up your 2nd and 3rd gems? Or would it be better to level 2 or 3 LGems at once, thereby boosting your character’s abilities across a broader range? I suppose it might matter how many good socketed jewelry slots you had, though in my exp it’s worth trading almost any jewelry affix for the bonus provided by even a Rank 0 legendary gem.