increase in crit equals how much %mainstat?

been trying to figure this out, and i'd rather have it doublechecked, i'm looking for the ratio of %main stat increase to equal an increase in critical damage from 1.65 to 1.8, i'm ignoring secondary stats.

x=main stat
y=amount of potential
z=added potential

this is what i came up with, but for some reason i can't help but feel there's something wrong here:

1.65*(x+x*y)=1.8*(x+x*(y+z))
1.65x+1.65xy=1.8x+1.8x(y+z)
1.65xy=0.15x+1.8x(y+z)
1.65xy-1.8x(y+z)=0.15x
x(-0.15y-1.8z)=0.15x
-y-12z=1
z=(y-1)/12