General

@Doutei dont forget the guild success rate of 4%. But yea what they said I guess. If you have max diligence and the guild success rate, 70% traces(84% with dili) on, for eg a fafnir with 8 slots(assuming you won't spend mesos on hammer) will give 30 or 35 atk whereas the 30% trace (now 44%) will give 21 or 28 attack.

Now if this is temporary gear, just use 70% and get to 10/11 stars (after 10 it's like 5-7 times more expensive than pre 10 stars). End game gear is icoging the armour and prime scroll/15% trace the weapon. Make sure u max out your diligence because it'll help in the long run. Use the leftover money to get a tyrant or gollux set maybe.

you will want to icog, which is completely possible with events. The only issue is
the ratio of att/stat you will have. But if you will be using spell traces exclusively
then you might as well go with 30%, 70% is a lot cheaper though and you won't
have to worry about using slates but it won't be anywhere near as good as it could
and you won't get any attack from it. Star forcing really doesn't matter since it makes
no difference how you scroll it and it would easily be cheap enough to star force for
all your equips to at least 10 star

Cog them. You want the atk

just 70% it. You can always innocence it later if you want to 30% or chaos.

you can yolo 30% [+10 if uhave max dilligence] or do 70%[+10].
don't bother slating since it's a waste on these scrolls. so you'd waste almost 0mesos.
starForcing for 12-13 stars should be enough, and may cost you almost 100mil at worst per cRA equip.

in the long run you'd want to either prime them[600m+/scroll] or iCog them for atk[event shops-quest].