Action Points and exhausting defense is a 13 August 2015 article by Douglas Cole:
It was inspired by a reply to https://gamingballistic.com/2015/08/08/variant-d-rule-spend-hp-for-extra-damage/
Kalzazz mentions:
- The enemy made his attack by 3, I need to make my parry by 3 to have a shot.
Doug replied:
- The concept of trading margin of success of an attack directly for difficulty in making the damage roll isn’t how GURPS works, generally. When attacking, you make the risks ahead of time.
He is right in the basic rules, although in the 3e GURPS Compendium I before the introduction of Deceptive Attack in 4e there was a concept of MoS automatically creating defensive penalties for Trained By A Master under Chambara Fighting which was removed by 4e's GURPS Martial Arts version
- Feverish Defense, which is 1 Fatigue Point to get you +2 to a single active defense roll (and suggests 5 AP per +1 to your defense roll in general)
This suggests using the default ratio of 10 AP = 1 FP when translating costs, even though burning 1 FP only restores 5 AP to the basic character.
Since HT 1 characters can have 1 AP, ratios above 1:1 take a lot of options off the table unless you allow people to go into negative AP the way they can AP or FP.