Is your DT a HM/dT or a VT? I wouldn't suggest crossing a VT to a PK if you're leaning towards the PK. VT will ruin the HM look on your PK.
DT x ST will produce ST, DT geno, Poor DT (unequal lobes) .... it depends on several factors. As 1f2f said you won't get DTPK in F1. You need to select the genos (PK with long dorsals) that suits your goal and inbreed. You will have DT in F2, but they might not be "perfect". You should get perfect DTHMPK in F3. If you don't, start over with other bettas (with better finnage).