A shortcut that I sometimes use for monster saving throws is 1d20+HD >= 16 (because who wants to be bothered looking at the fighter save matrix all the time?). The number 16 may seem arbitrary, but it is easy to remember one constant, and using that value has some nice properties with regard to the resulting probabilities by HD. You can also grant specific monsters bonuses or penalties on the fly as appropriate without jeopardizing impartiality, as long as the bonus is set before the roll happens, though I personally do not go more than +2 or -2.
Recently, I have been considering using ability checks directly for saving throws (I know, this is not new, Castles & Crusades, etc). The way I usually do ability checks is a d20 roll less than or equal to the stat in question, as described by Moldvay (page B60). Rolling under a single target number with no modifications in the common case is about the simplest resolution system possible (even though it does mean that rolling low is better, which some people find confusing). One potential downside to this method is that the target numbers are hugely variable based on chance during character creation.
Thinking about these two systems together (1d20+HD >= 16 and just using ability checks as saving throws) made me think that perhaps I could combine these two approaches without relying on improvised DCs. What I came up with does require using ability score modifiers, rather than just relying on the full score, but that is probably more familiar to most people by now anyways. It would probably work well with both the B/X bell curve progression (13-15 = +1, 16-17 = +2, 18 = +3) or the 3E linear progression (12-13 = +1, 14-15 = +2, 16-17 = +3, etc).
Here is how it works:
- PC save versus monster: ability check vs. 10 + monster HD
- Monster save versus PC ability: 1d20 + monster HD vs. 10 + PC level
- PC save versus dungeon hazard: ability check vs. 10 + dungeon level
For the purposes of these rules, HD values with additive components are always collapsed (so HD 1+1 is treated as 2). Using the ability modifier rather than the full ability score makes the chances less variable, addressing one of the issues with using the older roll-under style of ability checks as saves. The way I am thinking about this right now, the saves (as ability checks) would not improve with level (because I am interesting in seeing how that affects play), but level (or half level) could easily be factored in if you don’t mind one extra term. It might also be reasonable to have PCs gain some bonus for checks directly relevant to class (magic-users versus magic, thieves versus traps, etc). This could be operationalized as a flat bonus (perhaps +2), adding level to the roll (for games that have significant power gain with level increases), or rolling two checks and taking the best result (for games that prefer less numerical inflation).
Some examples. A magic-user casts a sleep spell on a group of orcs, which have 1 HD each. The monsters roll 1d20 + 1 (from the HD) versus a target number of 11 (10 + PC level). An adventurer touches a yellow mold, which squirts out a deadly cloud of spores. As written in the rules, a successful saving throw versus death ray is required for the adventurer to survive. Using this system, the save would be dexterity versus a target number of 12 (10 + yellow mold HD). As you can see from these examples, weaker monsters and lower level PCs have slightly improved chances compared to the traditional saving throw progressions, but that does not bother me.
At first glance, this looks something like the Third Edition approach to setting DCs, but I think this method is a worthwhile simplification, and it opposes known, objective values (ability modifiers, levels, hit dice) rather than needing to make any kind of determination about whether something should be a “hard” DC or an “easy” DC. This is a system that you can use entirely based on the HD value listed for a monster and the ability scores and level written on a PC’s character sheet. As such, it is easy to graft on to virtually any traditional-style fantasy RPG, as long as the game has ability score mods and hit dice listings for monsters.
This leads me even further toward the (perhaps slightly idealized) position that all you really need is HD and AC to specify a monster (along with a few interesting abilities, of course). And for monsters that are not humanoid, 10 + HD is a reasonable guideline for ascending AC as well.