Yuan-Ti,
you forgot the Black Company Campaign Setting MDT Rules.
Most of the MDT sytems presented so far a rather static. Either there is a static MDT or a static save. Both don´t take into account that, as the PC advance in levels they get better saves and deal more damage (and in tun get dealt).
I found, that the BCCS MDT rules work well for varying levels.
MDT: Con Score + 1/2 lvl.
Save: 10 + 1 per 5 pts. of Damage
More interesting is what happens, when the PC misses the save?