The equations used in this spreadsheet are based on NACA Technical Note 4197 (see
NACA TN-4197 which doesn't seem to be publicly available anymore) which presents a simplified method for calculating fin flutter velocity based on the method of Theoderson. NACA TN-4197 has confirming data up to "at least mach 1.3", so this may analysis may not hold up for higher mach numbers. NACA TN-4197 covers three different types of flutter: pitch-bending flutter, stall flutter, and torsion-bending flutter (or as the British say, "flexure-torsion flutter"). The flutter that we experience in amateur rocketry is torsion-bending flutter, that is, flutter due to the fins twisting and bending because they are not stiff enough.
A couple of difficulties. First, you have to know the fins aspect ratio which can be tough to calculate except for simple geometric cases. The solution here was to just calculate the simple geometric cases of tapered, clipped delta, swept delta, and triangular. You need numerical integration such as RockSim uses to calculate the aspect ratio of arbitrary shapes.
The second more difficult problem is finding the shear modulus for composite materials. Composite materials include carbon fiber, fiberglass, kevlar, and anything else that is epoxied together or to some sort of substrate. Plywood is also a composite as is G10.