Originally Posted by
karl_k0ch
I don't think that there's an error on your end, SisAmethyst.
If you are on a TR, having read a, say, +3 dex tome, the tomes apply as follows:
At level 3, your +1 tome kicks in before you get to chose your feat. However, 14 Dex is not enough to pick up TWF.
At level 6, your Dex is still 14, still not enough for TWF.
At the beginning of level 7, the +2 tome kicks in. Now your Dex is 15, and you can take TWF as the next feat.
At level 9, you can get TWF as a feat.
At level 11, the +3 tome applies. Your Dex is now 16, which is not enough for ITWF.
At level 12, you cannot take ITWF.
At the beginning of level 15, the +4 tome applies. You can now take ITWF.
At level 18, you can take GTWF.
This means that you're delaying your TWF progression to 9/15/18, if you are on a TR with a +4 tome. If you have a +4 tome, but you are not on a TR, the TWF line moves to 9/18/21. That's not very leveling-friendly.
However, starting with 14 Dex allows you to pick up the feats at 3/12/15 (on a TR) and 6/12/15 (if you need to read the tomes as soon as they become available).
Ranger PLs do not reduce feat requirements, and the TWF prereqs were not changed, as far as I know. It looks like 13 Dex is a good choice if you are at cap and have access to a +4 tome. I would not recommend this for leveling.