The proficiency loss uses a ladder points system. Every time you pass a class your character receives another ladder point.
so regardless of what class you are currently playing loss will be as if you are using the highest unlocked class.
The formula used for loss is
Code
Math.floor(current prof/(10- ladderpoints))
my Samurai has 2507 points into my current level of club and only 1 ladder point
if i were to die i would loose 278 points of proficiency
2507/9= 278.555555556 rounded down
TLDR:
Alchemist = ~33%
Headhunter = ~ 25%
Warlock = ~20%
Ninja = ~16%
Monk = ~14%
Paladin = ~12%
Samurai = ~11%
Feeder = ~10%