This is my general impression (no facts involved), if Runewaker used dice (random number generator)
+1 guaranteed
+2/+3 D6, 1-4 pass, 5 fail, 6 drop
+4 D6, 1-3 pass, 4 fail, 5-6 drop
+5 D6, 1-2 pass, 3-4 fail, 5-6 drop
+6 D6, 1 pass, 2-3 fail, 4-6 drop
+7 Guaranteed
+8/9 1-4 pass, 5 fail, 6 drop
+10/11/12 1-3 pass, 4 fail, 5-6 drop
+13 Guaranteed
+14/15 1-4 pass, 5 fail, 6 drop
+16 1-3 pass, 4 fail, 5-6 drop