I didn't look up it very carefully but I guess you should change "if (cItem.LegendaryGemRank == 50)“ with ”if (cItem.LegendaryGemRank == 30)“ to get what you want
Open AcceptTieredRiftRewardTag.cs
search and edit
"var rankedGems = SelectableGems.Where(g => g.LegendaryGemRank > 0 && GRiftLevel-g.LegendaryGemRank>0).ToList();"
7 means 70%
0 means 60%
-3 means 8%
-2 means 15%