The B&S NA2 is 20%...

A few things first...don't use pt when using NA2. NA2 is only to be used if you're using pd only. You can, of course, use pt + pd together, but then don't use the NA2. Use solution FO #2 for your contrast in that case.

With a 12 drop formula, as noted above in your post, one drop of NA2 is way too much. You need to dilute the NA2. Get a couple of dropper bottles, take half of the original 20% out and put it into a new bottle and add the same amount of distilled water. This new bottle will now be at 10% solution. Now take half of THAT and put in a fresh bottle along with the same amount of distilled water. This bottle will be at 5%. Now again, take half of the 5% and put in a new bottle with the same amount of dist water. This bottle is at 2.5%. One drop of 2.5% should be all you need for a decent neg made with 12 drops. That's a good amount to use for a 4x5 print...

Now that being said, you should start with pd, no pt, at first. Keep your FO and pd amounts equal. 6+6, and no NA2. Expose your wedge for the time it takes to merge two steps into black. That is your exposure time. It is your bible. If anything changes in your workflow (paper, neg material, developer, etc) do the step wedge test for basic exposure time again...

Needless to say, your wedge must be made in the same manner as your negs...and a lot depends on the density of your negs. Your negs must have a high DR and block UV effectively.