You might have use of this FAQ for various formulas, including Atma Weapon and Valiant Knife formulas: http://www.gamefaqs.com/ps/562865-final-...faqs/13573
I've pulled the Atma Weapon section from it and pasted it here.
After step 9 in damage calculation, the damage is modified as follows
step 10a. damage = damage * level
step 10b. damage = damage * ((current HP / 256) + 1) / ((max HP / 256) + 1)
step 10c. damage = (damage / 64) + 1
Based on step 10b, it appears that damage is basically calculated on a percentage of your hp.
Though, Vigor still has just as much of an effect as it does on every other weapon, for what it's worth.
I've pulled the Atma Weapon section from it and pasted it here.
After step 9 in damage calculation, the damage is modified as follows
step 10a. damage = damage * level
step 10b. damage = damage * ((current HP / 256) + 1) / ((max HP / 256) + 1)
step 10c. damage = (damage / 64) + 1
Based on step 10b, it appears that damage is basically calculated on a percentage of your hp.
Though, Vigor still has just as much of an effect as it does on every other weapon, for what it's worth.
The only thing harder than finding a needle in a haystack is finding a haystack in a needle. 