Ah, okay. This might be a design problem with no perfect solution.
In Cthork, with player vs NPC rolls, you typically just have the player roll against standard difficulty (DR 12). You can also apply penalties or bonuses based on weather conditions, the skill of the opposition, etc.
Those 'subtract from 20' values are for if an NPC needs to use a skill on their own. For example, Spot Hidden on a dingy room, or Climb up a cliffside.
That said, calculating 1 DR for every 5% of the NPC's skill definitely also works, and your method gives a lot more nuance to opposed rolls. I think GMs could use either.