It sounds like you've done everything right.
Remember, ferric nitrate is a chemical reaction. It reacts with the natural, and applied tannins in the wood. If chemically, it has a lower concentration of tannins, the reaction will be much less.
That's why we use test strips. If you don't have scraps, it's a $#@* shoot.
It won't get probably won't get darker. It's stain from here.