You still need to be able to ballpark it. The line between really hard and TPK is razor thin.
But being able to ballpark it is fundamentally different than being able to do it expertly.
I also disagree about it being razor thin. Poor dice, poor tactics (bonehead moves) and poor party composition for a specific monster can turn a moderate difficulty encounter into a TPK even with the best "balancing", but the line between high difficulty and a likely TPK is quite vast if we are assuming good play and average luck.
When TPKs happen it almost never is driven primarily by encounter difficulty. The factors that cause TPKs in order are:
1. Low level PCs where individual die rolls have more impact.
2. Poor tactics/play by the PCs
3. Statistically poor luck, off by a standard deviation or more (either extremely great monster rolls or extremely poor party rolls or both).
I would say at least half of TPKs I have seen in 5E have all 3 of these things, every one I have seen has at least one of those things and not all of them had particularly difficult monsters.
Edit: Two weeks ago our party of 5 level 2 PCs went into a fight against 20 Gnolls, 9 Dire Wolves, one Ogre and won. We are playing in a sandbox and walked into an area intended for 4th level PCs. We then made things worse when the alarm got raised and went from fighting 4 Gnolls on round 1 to fighting all those things on round 2. This is way, way above high difficulty in terms of balance, it is off the charts difficult and by all rights we should have either fled or statistically taken a TPK (and a three of us were very close to fleeing and leaving the two Barbarians behind).
Last edited: