For those who don't know, the way the leg upgrade gems work is as follows:
1. You will find all the gems pretty quickly, there are 13/14 of them or so. You won't find the same one twice unless you move it to another chars inventory. If it's in your inventory or stash it won't drop again.
2. The boss of the goblin realm drops a gold find leg gem, he is the only one who does they don't drop from grifts.
3. In order to have a 100% chance to upgrade a gem a level, your grift level must be +10 levels over your current gem level.
4. You get 3 chances to upgrade a gem, picking upgrade removes your grift token.
5. Based on the 2 points above, a rough formula for what level you need is [gem level] + 3 + 10. So if you have a level 5 gem and you want to 100% upgrade it 3 times, you must be on grift 5 + 3 + 10 = 18.
6. For each level below +10, you lose 10% chance to upgrade. So if you have gem level 2 and grift level 10, you will only have 80% chance to upgrade ((10 - 2) * 10).
7. There is an exception to point 6. As long as your gem level is lower than your grift level you will never drop below 60% chance to upgrade.
8. Not 100% on this, but if your gem level is the same or above your grift level, you will have a very small chance to upgrade. for example gem level 31 and grift 30, will only give you 30% chance to upgrade. Each level above lowers the chance by 50%.
Therefore for efficiency of upgrades I recommend only going up to the level you need to get 3 * 100% upgrade chances, unless you want to challenge yourself or whatever, because higher levels don't help you upgrade faster.