The answer to your question is yes.
Fanatic can choose a number between 0 and 5 to deal to herself, the actual damage she takes will depend on the cards in play. Buffs like Galvanize and Inspiring Presence will add to this number, and debuffs like Undaunted will subtract from this number.
Once you have determined how much damage Fanatic deals to herself, you then dish out an entirely new set of damage via Sacrosanct Martyr, which will include adding and subtracting the relevant buffs and debuffs again, possibly including the same buffs you added to the original damage.
For example
Fanatic chooses 5 damage for Sacrosanct Martyr
Legacy has used Galvanize this turn
Fanatic has Undaunted out
Obsidian Field increase all damage by 1
The Contract (a villain card) increases damage dealt to Heroes Targets by 1
Fanatic will deal herself 5+1+1+1-2=6 damage (choice+Galvanize+Obsidian Field+Contract-Undaunted)
Fanatic will then deal out 6+1+1=8 (Radiant damage this turn+Galvanize+Obsidian Field) damage to a target of her choice (The damage is not increased by The Contract, as she is choosing not to hit a Hero target with his attack)
This example presumes she has done no Radiant damage to herself during her turn via any other cards.
Fanatic can choose a number between 0 and 5 to deal to herself, the actual damage she takes will depend on the cards in play. Buffs like Galvanize and Inspiring Presence will add to this number, and debuffs like Undaunted will subtract from this number.
Once you have determined how much damage Fanatic deals to herself, you then dish out an entirely new set of damage via Sacrosanct Martyr, which will include adding and subtracting the relevant buffs and debuffs again, possibly including the same buffs you added to the original damage.
For example
Fanatic chooses 5 damage for Sacrosanct Martyr
Legacy has used Galvanize this turn
Fanatic has Undaunted out
Obsidian Field increase all damage by 1
The Contract (a villain card) increases damage dealt to Heroes Targets by 1
Fanatic will deal herself 5+1+1+1-2=6 damage (choice+Galvanize+Obsidian Field+Contract-Undaunted)
Fanatic will then deal out 6+1+1=8 (Radiant damage this turn+Galvanize+Obsidian Field) damage to a target of her choice (The damage is not increased by The Contract, as she is choosing not to hit a Hero target with his attack)
This example presumes she has done no Radiant damage to herself during her turn via any other cards.