First of all, I'd like to point out that observations like yours can only show what might be possible. You can't take a couple of duels and use them to extrapolate the formulas used in dueling. Secondly, if you happen to have missed a possible factor, such as in your first example, then your theory will be off.
1st fight :
I have 239 shooting 47-79 weapon, Adversary have 68 reflex.
I have done 112 & 158 Damge in hand, 112 & 132 Damage in shoulder
So : Shooting is not a "+" Damage skill for 1 SP = 1 Damage.
The post above by bonez, suggests that not just reflex would be considered, but that against a shooter, toughness would also come into play at 50% of it's value. You only give the defender's reflex here. Perhaps you've missed an important factor involved.
2nd fight :
Adversary have 106 power (skill for melee damage, is the name correct?) and a 15-85 Weapon. I have 30 toughness.
Damages : 89, 123, 136, 243 in head.
Not sure what your point is. Is just the final hit in the head, or all in the head? If you were to add the vigor and weapon damage, assuming a very high damage of 201 points, then consider +50% for a head shot, the 243 would be possible.
3rd fight :
I have 65 Dodge, and 0 Tactics. He have 30 Aiming and 68 Appearance.
I change side each round ... he hit me 8 time in the hand !
This is possible under the rules, if all of the random values based on your dodge were low, and his were high. Are you saying that he could hit you in the hand 8 times over the course of many duels? Or is this a one time thing? Might just be a fluke. You say you change sides (dodge?) each round. So were you dodging the hand that he hit each time? Or were you dodging the hand he wasn't shooting at? That would make a difference.
I have played a similar configuration, low tactics and low dodge. Then when I kept my tactics down, but increased my dodge, people began to miss me much more often. So I would say my own experience is that just increasing dodge alone makes a significant difference.
Btw... how do you know the skill point values of the characters you dueled?