The rules that gives a bonus point for having the king of trumps is quite strange. But it is a good idea to balance the power of having the king of trumps by forcing to show it to the opponent.
So, I'm using this scoring system:
a. 1 point by the player winning at least 3 tricks b. 1 extra point by the player winning all 5 tricks c. 1 extra point by defeating a vulnerable player d. -1 point by NOT marking the king of trumps in a hand before the first card is played e. -1 point by NOT following the suit correctly