Lanefan
Victoria Rules
Trial and error is the answer, yes.Of course, then you are giving the DM absolutely NO guidance on how to build encounters. How many orcs are appropriate for a low-level party? 10? 100? What about bugbears or gnolls? What about giants? I guess the DM could "figure it out" via the Guess and TPK method.
Beyond that, all it needs a bit of perhaps-intentionally vague guidance suggesting the DM compare the total party levels vs the total HD of the foes they're facing with advisement that things might get messy if Foes HD > Party Level Total.
And provided the players play their characters with a bit of a sense of self-preservation, TPKs are (or should be) vanishingly rare in any case unless the DM really screws up. All it needs is one survivor and the party can continue.