There is a good guideline for understanding the "X" designation in fly tippets... the rule of 11. These ratings were probably developed in the days of twisted gut leaders but they are useful in that you can find the leader you want quickly regardless of brand or pound test for purposes of matching your leader and fly with a tippet.
Rule of 11...
0X == .011
Subtract .001 for each increment, thus 1X = .010
2X = .009
3X = .008, etc... see chart below
Note that the X comes to the front when you exceed the 0X standard, in other words put the X in front if the size increases from 0X.
X1 = .012
X2 = .013