Tippets are not identified by breaking strength.  They are identified by their diameter in thousands of an inch.  No matter who the manufacture is a 4X tippet should be .007" or it isn't 4X. 

This relates to the old process of drawing gut through ever smaller holes to obtain a final diameter for a gut leader.  One draw was 1X or .010". Three draws resulted in a 3X rating or .008".  The result of this consistent diameter measurement is that you always know you can get 7X tippet through the eye on a size 20 fly.  

The whole thing is governed by the rule of ELEVEN.  The X rating and the diameter equal eleven.  A 3X tippet is .008" and 8 plus 3 equals eleven.  A 7X tippet is .004" and 7 plus 4 equals 11.  See what I mean?

So, is there a chart or something that matches fly size and leaders? Yes.  Do you have to carry this chart around? No.  Not if you remember this easy formula and use it for a guide.

Fly Size divided by 4 plus 1 equals the X number you should use

Or you can memorize this chart:

X Rating Diameter Fly Size
0X .011 2, 1/0
1X .010 4,6,8
2X .009 6, 8, 10
3X .008 10, 12, 14
4X .007 12, 14, 16
5X .006 14, 16, 18
6X .005 16, 18, 20, 22
7X .004 18, 20, 22, 24
8X .003 22, 24, 26, 28
For example take a size 6 hook, divide 6 by 4 and you get one.  Add one to that and you get 2. So for a size 6 hook use a 2X leader.  Of course the chart says 1X or 2X will work - which is right?  Well both are.  The formula is a guide to get you close.  A lot of people make do with a spool of 2X, 4X and 6X tippet.  Check it out, 2X will do hook sizes 6, 8 and 10; 4X will do hook sizes 12, 14 and 16; and 6X does most everything else.  Does it matter which tippet you use? Really?  Yes it does.  You might get a 2X leader through a size 16 fly eye, but you won't get much of a dead drift. The tippet will drag.  Adjust your tippet to your fly and catch more fish.

In summary; To cast a fly well and to have it behave like a natural in the current or during a retrieve a fly should be matched to the appropriate tippet diameter. This chart or the formula give you suggestions on balancing the tippet and fly.  They are just suggestions.  The conditions, type of fish you're casting to and other factors should influence your choice. However, using either the chart or formula will get you in the ballpark.