Is there a guideline for encounter building? I know there's NPC difficulty ratings but I didn't see a section giving any kind of formula for calculating encounter difficulty.
There's no in-built encounter-building system, as every encounter guide I've encountered in a TTRPG has been wildly off the mark. But here's a rough guideline you can use as a scaffold:
==============================================================
An easy encounter should have a total Difficulty equal to the number of players. So 3 players would be three Difficulty 1 synthetic enemies, a Difficulty 2 and a Difficulty 1 synthetic enemy, or a single Difficulty 3 synthetic enemy.
--------------------------------------------------------------
An average encounter should have 50% more Difficulty than the number of players. So 3 players would be 4.5 (rounded up to 5) Difficulty 1 synthetic enemies, or a Difficulty 3 and Difficulty 2 synthetic enemy, and so on.
--------------------------------------------------------------
A difficult encounter should have twice the Difficulty as number of players. So 3 players would be six Difficulty 1 synthetic enemies or two Difficulty 3 synthetic enemies.
==============================================================
Just keep in mind a few caveats:
==============================================================
The math breaks down when you hit bigger groups due to the power of a player character vastly outperforming the power of a single point of Difficulty. If you have more than 3-4 players, you'll likely need to bump up these numbers another 25-50%.
--------------------------------------------------------------
A single combat will rarely be deadly, but the players will rarely face a single combat between repairs. This is a skirmish game; focus on chaining multiple combats together rather than trying to set up massive setpiece battles.
--------------------------------------------------------------
Marginal advantages add up faster than Marginal hindrances, meaning enemies will often gain +1d to their attacks. Each die is a 50% chance for a success, so even Difficulty 1 enemies will statistically get at least 1 success with a Marginal advantage.
--------------------------------------------------------------
Organic enemies act after player characters and rarely have more than 2 Health. Sometimes, they even go down with a single success. Use 2-3 organic enemies for every synthetic enemy when accounting for Difficulty. Squads are a great way to ensure you're not dealing with too much overhead despite the numbers.
==============================================================
Again, it's not playtested so this is all just theoretical. But I hope this helps as a foundation!