I did some research of my own so we've got a complete report of how things seem to work instead of 2 pages of posts pointing one's error to the other.
I am going to use a legendary (yellow) tier 2 armorsmith's hammer. Starting durability is 37.
Armorsmithing tier 2 cp: 300
Armorsmithing tier 3 cp: 60
Tanning tier 2 cp: 226
Tanning tier 3 cp: 222
First, I am going to craft 2 pieces of medium leather (tier 2). I'm hoping of getting 2 normal (x1) crafts so I can see if I lose 0, 1 or 2 durability (0 would mean crafting only rares or better reduces durability, 1 would mean 1 point per ap spent, 2 would mean 1 point per item crafted).
Got 1 x2 craft and lost 1 durability (currently 36). This rules out the 1/item loss but does not prove 1/ap or 1/non-normal craft. So, I am going to repeat the try but now I'm gonna craft 6 pieces of medium leather (costing also 1 ap), hoping for more than 1 x2 crafts. If I lose only 1 durability, it almost proves the 1/ap theory.
Got 3 x2 crafts and lost 1 durability (currently 35). Just to make sure 100% that it is really 1/ap I'm gonna craft enough leather to spend 2 ap. More precisely, I am going to craft 9 pieces of medium leather.
Got 5 x2 crafts and lost 2 durability (currently 33). This confirms that regarding same tier resources crafting, the durability loss of used tool equals the action points spent.
Next, I'm gonna try crafting some tier 3 items while having the tier 2 tool equiped. Since my armorsmith is incapable of non-normal crafts but my tanning is, I am going to test if it is because Crassus's cp was below 200 that he didn't lose durability, or if it is because the tier of crafting was not the same with the tier of used tool. Then I'm going to equip a tier 3 legendary tool and craft an armor piece to see if durability goes down or not (currently 50).
First craft is 1 heavy leather (tanning tier 3) where 1 durability was lost on my tier 2 tool (currently 32). It should be noted that inspite of tier difference, the x2 chance was shown as 59.40%, meaning that a tool's tier seems irrelevant to the resource's crafting tier. I'm gonna do some more testing on this later.
Next craft is 1 superb hard leather helmet (armorsmithing tier 3) where no durability was lost on my tier 2 tool (currently 32).
Next craft is 1 heavy leather with tier 3 tool. Lost 1 durability on this one (currently 49).
Last but not least, 1 superb hard leather helmet crafted with the tier 3 tool did not decrease durability (currently 49).
To conclude, it seems that regardless of the tool's tier, if one crafts an item of a tier where they have more than 200 points, the tool is going to work and lose durability equal to 1 point per action point spent.Now if you ask me, I could consider this a bug, since we seem to get normal bonuses by using crafting tools of tiers lower than our actual crafting. I would like to also test this both in armorsmith and in greater numbers but first I've got not enough cp in tier 3 armorsmith to test with tier 2 tool and second I already spent around 65 diamonds.
If you excuse me now, my batteries need recharging.