unlike weapon speed, and other things (
like shields, which are closer to what we're talking about), there's no clear cut-off point for armour. check the bots rather than the wiki for this stuff:
https://crawl.develz.org/info/index.php?q=aevpfor mottled the penalty is almost irrelevant. i train it to 5 and i think most people do the same or something similar. for heavier armours (like fire/ice/pearl) it's often better to just train the magic skills higher (and if you have many, stay with light armour). swamp dragon armour is usually not worth it if you're casting.
also: there is no "medium armour", evasion penalty, which includes mda, 1 is light. anything beyond that is heavy. it's mostly irrelevant, but the game uses this distinction for a few things (like ash skill bonuses? i can't think of anything else right now).