Margin of Success or Failure is a powerful tool, I only noticed GMs have run the system a long time. Any new GURPS GM can pick up this very helpful tool, but what I don't see alot is how to apply "Called Shots" or Harder Tasks with Greater reward.
In my open game system I tackled this and I believe it can apply to any other system. The basic rule is that in "Called Shots" the margin of success is penalty taken PLUS 1.5! Basically 50% greater than a normal feat.
So I'm a Cook in GURPS with skill-15 cook A, vs another Cook, B, with Skill-12. In a "Quick Contest", A rolls 9 with a MoS 6, and cook seeing what A is about to do chooses to impose a -5TDM penalty by choosing to do a harder feat and reducing his chance to succeed to 7! IF B succeeds his MoS will be (5*1.5) 7 enough to beat A if not, it would probably have a high MoF which seriously disappoint the judges.
Margin of Success simplifies the system and makes the Roll Matter more.
In my GURPS game instead of rolling the "Dodge" Rules in a combat with 6 players in roll20 I asked them to make DX rolls to dodge an explosion they all could see coming. I allowed for the +3 from Dodge and Drop. The margin of success + Cover which adds an MoS of 5 divides the damage by the Margin of success.
Everyone who made the roll divided 25 burning damage by their MoS. One badguy failed by 5, so even with cover it was not enough, he took 25 burning damage straight. (Fright Checks seeing someone run around with 3 degree burns and still burning) for a couple of seconds.
Margin of Success Penalty
Sometimes we are using a very unusual skill for a particular task. Lets say I use Pyschology or Savoir Faire for Diplomacy checks, . other than the default penalty of about -5 to -6 another penalty one can impose is a shallower margin of success. You can half the margin of success if you feel that the handicap is not enough.
In Social Situations, typically I like using Margin of Success or Failure as a way to temporarily or permanently improve reaction score of a target. Of course there are diminishing returns for some strategy - Savoir Fair goes only as far as courtesy and thoughtfulness, while others are more effective among various characters. A simple rule of thumb is to half the MoS for the preferred "language" (the prefered social strategy) and maximise the improvement by one level.
Note that from Indiferent to positive, with enough time someone can convince another person, if it is reasonable and within their acceptable tollerances. So ideally there is not enough time OR there is other conflicting individuals who is against you, if you want a challenging social situation.