The question is actually in the subject. I have few crafter tiwnks and I try to increase probability of dropping master writs. Is it worth to learn nirnhoed trait on all of them or not? Does anyone have some valid statistics on subject?
Number of total motif styles (ones with individual pages) learned are the best chance of increasing master writ drops, I believe.
I don’t know if anyone has stats on nirn but generally I think most players do all nirn on main toon only and just the two traits (potent and fortified) on others for the achievement.
Khajiit Stamblade main
Dark Elf Magsorc
Redguard Stamina Dragonknight
Orc Stamplar PVP
Breton Magsorc PVP
Dark Elf Magden
Khajiit Stamblade
Khajiit Stamina Arcanist
When ZoS introduced the master writs, they used the phrase "achievement linked" to describe what would have an effect on the drop rates.
Thus, I'd conclude that for alts you'd only need to get the achievements for researching a nirnhoned weapon (potent) and a nirnhoned armor piece (fortified) to increase the chance of master writs.
Same as how you need to learn all 14 pages of a motif for it to have an effect on your drop rate, which completes the achievement associated with it.
Motif knowledge appears to be heavily weighted towards the first dozen or so you learn having a larger impact than those learned after those first ones.
I think the only exception to the "achievement linked" is provisioning, where they said it's the number of purple and gold recipes you know, and there isn't an achievement for that.