The new STR cap? There's no new STR cap, the formula has stayed the same. Are you talking about the max of fSTR? Meaning, how much STR above the targets VIT before you cap out on the bonus from the difference?

"When STR-VIT value is high, fSTR can be approximated by ((your STR - monster VIT)+4)/4."

And the upper and lower fSTR values are based on your weapon's rank. Your Weapon Rank is calculated by taking your weapon's base damage and dividing by 9. Once you have that, your lower fSTR value is that value, and your upper value is that value +8.

Let's use the Masamune (90) as an example:

floor(114/9) = 12 <= The real answer is 12.666666~ but since we floor it, it's value is 12.

Now that we know Masamune's Weapon Rank, those with a Masamune know their fSTR can be:
Lower: -12 <= (-Weapon Rank)
Upper: 20 <= (Weapon Rank +8)

Solving backwards with the equation above regarding STR and enemies VIT, you can see that to hit the maximum of fSTR 20 you need to have about 76 more STR than the enemy has VIT. This will change depending on what you fight.

I sure hope I answered your question. It was worded a bit hard to understand for me. If I didn't solve your question I'm sorry!