I think I heard some else say otherwise but I feel certain that the second hit of both strings have the same range and that 21 is almost useless in punishing.
Havn't got the new patch so can't test it but just looking at videos dash to 12b1 should surely work. B34 has more range and might not need it and if all else fails then maybe f2b1? I heard that punished KL's spin which was ultra hard to punish when the game was first out.
I was wondering with the new push back if 1,2,b1 would be usable anymore. If not, b2 has a lot of range as does b3,4 (and this is also quick). If b2 works, you'll actually net the most damage from starting with that.