At what point should I save my failstack for higher attempts?

When should I stop using a failstack and save it for higher attempts? I've been trying to tri my fucking nouver and I got to 80 from 44 then extracted it and now I have a 70 FS (from 42), when do I pull the plug and go back to a lower enhancement level?

Also for PRI and Higher, does all gear have the same enhancement chances/failstack success increase, or is it like it is for 0-15 where it's harder to enhance boss gear?