[@Archangel]

Oh dear, that's not quite right. Here, let's walk through this:

We start as:

STR - 13
DEX - 13
CON - 13
INT - 13
WIS - 13
CHA - 13

First, we remove the points you wanted:

STR - 13
DEX - 13
CON - 13
INT - 13
WIS - 11
CHA - 8

In total, we now have 7 points to spend. We only need 5 to do what you wanted, so I'll put (as an example) the other 2 in DEX.

STR - 13
DEX - 15
CON - 13
INT - 18 (1:1)
WIS - 11
CHA - 8

Now that we have that covered, this looks like the beginning of a Wizard build, so I'll use that as my example. (naturally, I don't know what you're actually doing, but this is an example).

We'll take 5 points from STR and 1 final point from WIS, since it isn't doing anything, giving us 6 to work with. Then we will plug them all into DEX.

Now, here's where the 2:1 comes in. Since we ALREADY have a stat that is higher than 15, any OTHER instance of increasing a stat beyond 15 now uses 2:1. Since we have 6 points, we can make DEX another 18!

STR - 8
DEX - 18 (2:1 past 15)
CON - 13
INT - 18 (1:1)
WIS - 10
CHA - 8

And there you have it. The final step is to add on top any race bonuses you have. Hope that helps.