[BX/OD&D] Towards a single save

 In my game of Trow Fortess, I don't use the five saves. I only use a d6 pool and two saving throw types (plus an special one at zero hp)

The types are the Easy (Death Ray, Poison and Paralyzation) and the Difficult (Spells including wands, Triggering Traps and Dragon Breath). Their chances by level are based on BX's Death Ray and Spells saves, respectivelly; and there is nothing in between (full spread of chances here)

I am thinking on trying OD&D at some point; but getting back to the classic 5-saving throw system and consulting charts is not appealing to me anymore (It's specially painful to check saves for monsters and having them referred to like "as fighter 4" instead of a fucking number). If you follow this blog, you know I usually take an effort to eliminate rules and charts that I feel are redundant, or just do not offer enough reasons in exchange for their cost.

So I started wondering if I could keep a single save number that scaled with level; and then, with advantage or disadvantage mechanics, cover my two save types mathematically faithfully. The answer is yes:

The two rows above are the fighter's saving throw progression for his best (death ray) and worst (spells) saves. These numbers are the same for OD&D and BX, saving me time because I had already done that calculations.

The third row is the Spells saving throw, rolled with advantage (2d20 keep best). As you can see, and without counting the unimportant normal man's saves, it follows the Death Ray saving chances with a maximum deviation of a 5% at level 10. This allows me with a clear conscience to use the spells save as the baseline single save, use it for the hard saves (spells, traps and dragon breath) then giving advantage for the easy saves (death ray, paralization, poison)

(PD: Lets ignore the fact that elf saves fuck this proportion completelly)

The fourth row is the opposite: disadvantage (2d20 keep worst) on the Death Ray numbers. The mathematical probabilites deviate from the original Spells' ones a little more, and the psychological act of rolling an easy save with disadvantage feels worse than rolling the hard one with advantage, so the previous method feels much better in both senses.