There are so many variables to consider in answering this question that in my opinion the simplest possible calculation is likely to give as good an answer as any other.
With that in mind:
Anna was carrying a single reefed mainsail and the smaller self tacking jib when the squall hit. The crew’s first response to the squall was to initiate reefing the jib and in doing so the jib outhaul, which is essentially the sheet, was let go most of the way, allowing the jib to luff. For the purposes of this simplistic calculation then, let's assume the jib was luffing and not contributing any side force.
The maximum righting moment of the boat is the product of the vessel loaded weight (approx 28,000 lbs) multiplied by the distance from the center of gravity to the heeled center of buoyancy (11'). This equals 308,000 ft/lbs of righting moment. To overcome that stability what force needs to be applied to every square foot of mainsail?
The mainsail area with the first reef tied in is 738 sq/ft. Its center of area is about 33' above the waterline. To equal the maximum righting moment of 308,000 ft/lbs, a force of 12.65 pounds has to be applied to every square foot of mainsail. (738sq/ft x 33' x 12.65lbs= 308,000 ft/lbs)
Anna had been sailing to windward and the mainsail was sheeted in fairly hard. Then the direction of the wind moved aft in a gust so Anna was sailing with the wind near the beam. Again for the purposes of the simplified calculation we can assume the mainsail to be a flat plate with the wind acting on it at a 90 degree angle.
Using the standard formula for the dynamic pressure of wind*, a force of 12.6 lbs/sq ft is obtained at a wind velocity of 103 feet per second, or 61.8 knots (70 mph).
Given all the fudge factors and unknowns that would need to be applied to refine this basic calculation, it makes sense to view it as the middle of a range that could easily extend 5 or even 10 knots to either side. But interestingly, Anna's crew reported seeing 62 knots on the wind meter moments before capsize. So maybe it isn't too far off the mark.
Running the same calculation for a double reefed Atlantic 57 mainsail yields a wind-to-capsize velocity of 83 knots (94 mph). I am sure these calculations are increasingly flawed at the higher wind velocities because I did not account for the drag of mast, rigging and hulls. However as a comparative number with the single reef calculation it does illustrate the fact that as the sail area is both reduced and lowered the boat is able to stand up to significantly more wind.
*The standard formula for the dynamic pressure of wind is:
F = 0.00119 x Va(squared) x Sa x C
Where:
F is the dynamic pressure of wind, measured in lbs/sq ft
Va is the wind velocity in ft/sec
Sa is the sail area in sq/ft
C is the aerodynamic force coefficient, typically taken as 1