But in the spirit of Fighting Fantasy, if you are going to roll another die for damage, why not make the rule as simple as possible?
Rolled Damage for Fighting Fantasy
When a hit is achieved, roll one die. This is the number of STAMINA points the wounded character will lose.
Heavy weapons (two-handed swords, troll clubs) add 1 point to the total STAMINA loss.
Light weapons (goblin swords, short-staffs) deduct 1 point from the STAMINA loss.
Light armour deducts 1 points from the STAMINA loss.
Heavy armour (like plate or a full mail hauberk) deducts 2 points.
A shield deducts one more point from the STAMINA loss if the character carrying the shield makes a successful test for skill.
(The GM may apply the protective value of armour (1 or 2 points) as a penalty to any test for skill involving agility or speed.)
A successful test for luck will cause a 1 point hit, if wounded, or a 6 point hit, if attacking. There is no penalty for failing the test for luck: a hit is bad enough.