I agree with @The_Merlitron the GM is narratively describing the action, it’s all upto the players to decide how they react, do they want to dodge, parry, block or something else? The focus is on the players choice and just let them roll to resolve.
At our table we’ve just abstracted a lot of the modifiers and math in favour of a faster paced game, yes we’ve lost a level of crunch but the action moves along much faster in play.
As always, the GM has dials to play with…
- Set an environmental difficulty, the “room target”, how stressful is the current scene? plain stone dungeon room with no hazards 10, rickety bridge crossing 12, stepping stones across boiling lava 15.
- Set the task difficulty and just use abstracted factors, to use an example from another post, a single goblin with a rusty knife Easy, a few goblins with spears Normal, goblin champion Hard.
It doesn’t need to be any more complicated than that.
With player facing rolls, the players need the crunchy stats to determine their success, but not everything in the game needs that level of crunch. We’ve learnt to just boil down everything and get on with the game.
Is that monster agile, darting around really fast making him Hard to hit? …Nobody really cares what it’s +Dex modifier is, only the probability of hitting it.
To get back to defend rolls, players roll +Def to beat the room target, factor in unique monster features like are their competent fighters or difficult to defend against lashing tail attacks? then make it Hard roll, simple as that, don’t over think it.