Simulating Pack size

No, no,
This won't work the way your thinking it will with a trash bag.
You have to factor in that a circle will hold more gear than a rectangle.

For better results, what you need to get is the circular diameter of the bag and multiply "PIE" by half the diameter across the circle "R" (squared) to get the cubic inches.

Then multiply that by the depth you need to reach the desired 4200 ci.

You would then need to cap the bottom with out any taper, (so it is still circular.

Example: A 20" diameter circle

Half the distance is 10". So 10" squared = 100 X 3.14 gives you 314 ci per inch of depth of the circle.

So a circle would even fit more gear than if the size of the bag was 20” x 15” = 300 ci per inch of depth.

Styrofoam popcorn works far better than this method.

You lost me on the part about taking off the last 12 inches and taping the sides…

Why don't you just pack your gear inside the garbage bag and see how much they fill up? If, for example, the 158L bag is 50% full, then it's 79L or 4,821 c.i.

Thanks for the help Aaron, I think Ill do this:

Ill create a 10 inch diameter circle and then create a rectangular piece of 31.4 * 28

This would give me

5^2 * 3.14 * 28 = 2198cl

Thanks, I should have went with the volume of a cylinder formula.

There is more
It depends on how a manufacturer measures the pack size. Often a lot of that is derived from using the theoretical holding capacity of the external pockets.
Just get hold of a 40 L stuff sack, fill that and hold it next to a 2400cl backpack. Even considering the very valid point from Aaron, you will see the difference.
Franco