Okay for the example ditto has 30/31/30/31/31/31, the other parent has 6IVs. The goal is to get the offspring with the same IVs of ditto. With destiny knot the IVs inheritance is: 0-5, 1-4, 2-3, 3-2, 4-1, 5-0. The total combinations of iV spread inheritance is (6 (C 6,5) 30 (C 6,1*C 5,4) 60 (C 6,2*C 4,3))*2 = 192
Each IV type of inheritance gives different chance of getting the target spread.
1-4: the IV inherited from ditto must of be from hp or defense, the second 30 iv is up to luck. 2*5(=C 5,4)/192*1/32
2-3: inherit both 30 ivs from ditto, required to roll a 31 IV. 1*4 (=C 4,3)/192*1/32
the chances of the "all 30" ditto stop here and are 7/3072 while the HP ditto can afford to pass down just one 30 Iv and let the other be decided by luck, so 15(=C 6,2)/192*4(=C 4,3)*1/32
3-2: similar to the case above 2 of the 3 IVs inherited from ditto must be hp and attack, the other 4 IVs are 31 and for the last one you must be lucky. Otherwise just get one 30 IV and get 30 in the other stat with luck.
4 (= C 4,1) *3 (=C 3,2)/192*1/32.
10 (=C 5,2)/192*1/32
4-1: 2 of the four IVs are 30, other three IVs are 31 and the last one is up to luck, or get just one 30 iv from ditto and roll a 30 IV in the other stat
6 (=C 4,2) *2 =(C 2,1)/192*1/32.
5(=C 5,4)/192*1/32
5-0: inherit all the IVs from ditto, the last IV needs to be either 30 or 31.
6 (=C 6,5)/192*1/32
summing up all the chances that's what I get: 119/6144