I'm trying to run the newly released Against the Slave Lords compilation using the D&D Next rules and I've run into a problem with converting Saving Throws. How do I go about converting something like "Save vs. Breath Weapon" into a DC? I can make a reasonable guess about which ability modifier will be used for the roll, but how do I decide what the DC is? Should it still be class-dependant like it was in AD&D, or is there a static DC for each type of Save from the original set (Breath Weapon, Petrification, etc)? I haven't been able to find anything official from Wizards on this and to avoid exhaustively crunching the numbers on the old "Save matrix", I was hoping someone knew something I didn't (or has a house-rule I could use).
It's important when tackling this to notice that D&D Next saves' math are tied to the effect, while AD&D saves math—despite the names being that of effects—are actually tied to class and level. That means that there's no way to crunch the AD&D math to find a conversion formula—the numbers are representing different things.
As is often the case when converting between editions with different fundamentals, this can only be approached as a "re-imagining" of the source material. Use the D&D Next guidance on setting save DCs as if you were creating this adventure from scratch yourself. (After all, for adventure elements that are non-translatable, that is actually what you're doing.) Imagine the danger and the degree of challenge it poses, and use your judgement to choose a challenge level that matches that, from the range of Trivial to Nearly Impossible, to get an appropriate saving throw DC.
If you can determine the appropriate stat to use in most circumstances, you have already fought half of the fight. It takes a bit of analysis to decide how hard it should be to save against, but use 3rd and 4th edition D&D to get a rough idea if you can't just glean it yourself, assuming you have played them as well.
Overall, some generalizations can be made to help the decision-making process as well.
Spells have a standard ruling to determine how hard they are to resist, so that part is normally easy to translate. Base spell-like abilities off of the spells that are closest to them if you can.
Sometimes you can guess where it's close to based on the context of what is doing it. A goblin wizard's Burning Hands will be far easier to dodge than a red dragon's breath weapon, and should have an appropriately lower reflex save DC.
If all else fails, decide if you want it to be easy, average, difficult, or impossible to resist, and fudge the numbers as you go. You might even decide based on the percent chance that their character, with their stats, should succeed, and let them roll based on that. Improvisation on these things takes care of it bogging down your game, but once you make a decision, keep a note of it and adjust fire for other players to stay consistent with your decisions.
So here's what I came up with. I took a look at the AD&D Saving Throw tables for both the first and second editions. In both cases all classes have the same basic distribution of difficulty except for Mages. It goes like this:
Fighters/Clerics/Thieves From hardest to easiest:
NOTE: For Thieves 4 and 5 are swapped
For Mages the order is such:
I figure that this difference reflects the fact that Mages are better at saves that effect the mind while the other classes are better at ones that effect the body. As such, I've come up with the following conversion