I'm not an experienced breeder, but I've been studying genetics at uni and this got me interested.
From what I understand DT is recessive to single tail. If you cross a DT with a single tail, you'll get nearly all singetail fry, but with about 75% of them carrying the recessive DT trait. Cross one of these babies back with a DT in an F2 and you'll get about 50% DT fry.
The DT apparently can help to increase the volume of the finnage, so I'm sure they'll be beautiful babies.
Crossing DT to ST that doesn't carry the DT gene will get you 100% STs, all of which carry the DT gene. Half moons are usually DT carriers though, so it wouldn't be unreasonable to expect 50% DTs.