I needed help with this some time ago, and it resulted in a massive debate that even, in some cases, degenerated into name-calling. Here is a summary. I hope it is helpful.
Normally, when doing dual deployment, you have to calculate for drag separation of the booster, and nose cone retention when the drogue charge fires. In your case, it is much simpler.
You need to know the weight of the NC (or the half above the separation point). You are trying to retain the nose cone after motor burnout, so that the nose doesn't continue upward while the booster or airframe starts to get pulled downward by gravity.
You also need to know the rocket's acceleration at burnout. You can get this from a sim. If you are not using OR or Rocksim, do a basic sim on ThrustCurve.org and use the maximum acceleration given for your motor.
Then take a worst case scenario in which, at burnout, the booster immediate decelerates to zero and begins to fall. The question is: how much force is required to retain the nose cone so that its momentum doesn't keep it going upward?
The equation to use is Force = mass (or we'll use weight) X acceleration; or, F = ma. You know the 'm' from weighing the NC, and the 'a' from your sim.
For our example, let's say the NC weighs 1.2 pounds, and the maximum acceleration is 8G. Subtract 1G from the acceleration, since the acceleration of a static velocity is 1. So, we get F = 1.2 x 7, or F = 8.4.
This means you need to apply 8.4 pounds of force to retain the NC. If you look online at the shear strength of whatever you are choosing to use for shear pins, take the minimum value, since you can't afford to assume that you will get a higher value.
6/6 nylon #2-56 screws have a minimum shear strength of 31 pounds. Since you only need to retain 8.4 pounds, you can see that one #2-56 nylon screw is way more than sufficient to prevent drag sep. of your NC.
With one screw, however, there is the chance of binding, since the pressure is applied unevenly. IOW, one side of the NC is free to fly, while the other is momentarily bound, potentially causing the NC shoulder (or coupler) to get cock-eyed in there.
There are many people who will simply go with one screw. Personally, I am comfortable going with two, while others will insist on three.
Thus, the next question is: when I do want the NC to separate (at apogee), how many pins can my BP charge break?
To answer that, use one of the online BP charge size calculators. Put in your diameter, tube length, and desired pressure, and it will calculate the required charge size and also tell you how many shear pins that size of charge can break.
Try the equation with your actual values. If you are using a high thrust motor like a VMAX, maybe your max acceleration is much higher, like 22 G. Even so, if your NC is 1.2 lb., you only need one pin.
Unless your shoulder is super loose, binding is very unlikely, and you can get away with one pin easily. But #2-56 screws are cheap, and if you are more comfortable with two or three, go for it.
A final note. FG is more than capable of cleanly breaking nylon screws, so you don't need any shim or anything. You will have to decide whether or not to tap them, or just drill clean holes and shove them in. What I do is tap the inner tube (in this case, your NC shoulder), and drill the outer hole slight larger. Then I screw them in. Many others will tell you this is a pain in the butt, and they like to tap theirs in with a tack hammer, or something.
Come back and let us know how it goes.