I agree with MrV. PK x VT= VT, shorter VT, sometimes RT, PK .... not sure about spade tail though. IMO it's difficult to get spade T from regular PK. You would most probably get oval VT which in an older age will bend and become the VT as we know them. It should be easier to get spades by cross breeding to wild bettas - the types that have spade shaped tails.
In an attempt to improve color some have crossed HM and PK to these wilds. They get the color but have difficulty in eliminating the spade shape of PK. I'm not sure about the HM result but I sometimes see HM with a rather spade shaped tail instead of the half circle. SO, I assume those wild spades are dominant over the splendens and their trait will be carried for generations. Thus if crossed to the VT, you should get spades easier...... as far as I know spade tails are not popular in my area and are considered a flaw.