The equation can be found here:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19620006811.pdf (Equation 1, simpler than I expected)

The moment of inertia of the rocket can be determined in different ways. As a first order approximation, I'd model it as a thick walled cylindrical tube:

https://en.wikipedia.org/wiki/List_of_moments_of_inertia
For a bit more precision, it can be modeled as the sum of its (simplified) parts. For those parts, that aren't centered on the rockets axis of rotation, the Parallel Axis Theorem can be used:

https://en.wikipedia.org/wiki/Parallel_axis_theorem
If you're using CAD software, like SOLIDWORKS, the moments of inertia can be easily determined, if the model is accurate.

Alternatively, the moment of inertia can be measured too. A simple approach is to wrap a line around the rockets circumference around the center of gravity. If you let it drop, so that it unwraps like a Yo-Yo, its potential energy doesn't only get converted into translational kinetic energy ("falling down") but also into rotational kinetic energy -> m*g*h = m*v^2/2 + I*omega^2/2. The relation between mass and moment of inertia will determine how fast it falls and how fast it rotates. Drag of the fins will influence the result a bit, but it should be close enough to get a reasonable estimate. In a practical test, two lines on both sides of the cg will be much easier than balancing the rocket on a single line.

Another approach would be to suspend the rocket from the nosecone from some kind of torsional bar or string to create a rotational pendulum and measure it's resonant frequency. For calibration, use some piece with a simple enough geometry, that it can be easily measured and calculated (e.g. metal rod or sphere).

Reinhard