Well that's true but original BX/BECMI doesn't have an official Assassin class to draw from so the whole class is essentially house-ruled. Now in the Masters set book it features rules to turn a Thug into a PC of sorts. And it's chance for Assassination is 50% but receives a penalty or bonus based on the targets level. Osric/1e essentially does the same thing. Even Advanced OSE allows the Assassin to apply a penalty to the target. So the only inspiration we have is either from:
Osric/1e has the Assassination tables but the math is the more or the less the same.
If an assassin gains surprise (see “Combat”) he or she may elect to assassinate the victim. The assassin need not roll to hit for an assassination attempt, which is a special attack resulting in a chance to kill the victim instantly and ensure damage even if the victim is not successfully killed. The percentage chance to kill a victim is 50%, with a bonus of 5% per level of the assassin and a penalty of 5% for every two levels of the target (rounding down; in the case of monsters, substitute “hit dice” for “level”). These numbers are approximate, for the GM should modify the assassin’s chances slightly upward or downward according to the circumstances—guarded or wary targets are less likely to be killed, and unwary victims are more likely to die. If the assassination attempt does not succeed, the attack still inflicts normal damage, together with any applicable bonuses. The assassin does not gain backstab multiple damage on a failed assassination attempt, even if the assassination attempt was made from behind.
Here's the OSE Assassin. The penalty mentioned grows as the Assassin levels.
Assassination (AS): When attacking
an unaware humanoid from behind, an
assassin gains a +4 bonus to hit. If the
attack succeeds, the victim must save
versus death with a penalty dependent
on the assassin’s level (noted in the table
opposite). If the save fails, the victim is
instantly killed, otherwise the assassin’s
attack inflicts normal damage.
Masters Set book Thug
If a prepared thug gains surprise, the victim may be slain with a single
blow, regardless of hit points. No Hit roll is made; instead, the
base chance of success is 50 % , modified by the difference in Hit Dice,
as follows: if the victim’s level is greater than the Thug’s, subtract 5 %
per Hit Die; if the victim’s level is less than the Thug’s, add 5 % per
Hit Die. If the thug does not gain surprise, a normal Hit roll is made,
and normal damage is inflicted if the attempt succeeds. A successful
hit may also require the victim to make a Saving Throw vs. Poison if
the thug is using a poisoned blade.
I personally like to draw from original sources when possible. That's just me. So I may personally change how the ability works or else as the Assassin levels his main ability will rarely work. As the Assassin levels he should get better at Assassinating since all his targets are improving in their saving throws.
