Just throwing this on here so I don’t forget it:
If you are a fan of opposed rolls in your game, and if you’re using different die sizes instead of bonuses, and if you have no clue what die size to use in a situation: Classic Traveller uses difficulty throws: the ref simply rolls 2d6, and that’s your target number. For game with different die sizes, roll 1 d20 and round to the nearest die size. That’s the type of die you roll as a ref.
A hero wants to shoot the knife out of a kidnapper’s hand who’s standing a good 100 yards away, half-hidden in the shadows. I roll a d20, get an 18, so the nearest die size is the d20. In my opposed roll against the hero I’ll use the d20.