How is the counterattack damage calculated?

same as if the enemy were to initiate

If basically adding everything up.

If they have weapon advantage its atk ×1.2 rounded down.
If they have disadvantage its atk *.8 rouned up.
This shows up is what shows up on the dmg calculator in the game

Specials are taken into account and added to the dmg u see. But otherwise you dont see the excact values but u can calculate for them

Do you have anything specific in mind or is this about the absolute basics (i.e. atk - def/res modified by the weapon triangle)?

The same as if the unit were to initiate;for a magic unit(tomes, staves or dragons), Atk - Res. For a physical unit(swords, lances, axes, bows, or daggers), Atk - Def.

Don't forget to factor in any spurs or drives that either side is receiving.