To my understanding, part of the reason life is able to exist on Earth besides its ozone layer is due to its magnetic field, which protects the planet and its life against solar winds and keeps the atmosphere from being stripped away. Now, I’m aware that Earth’s magnetic field varies a bit in strength at places as well (from 25 to 65 microteslas or 0.25 to 0.65 gauss), but that’s only against Sol’s winds, which vary from around 400 to 750 km/s depending on where the planet receives the flare.
Since many factors have to be taken into account for such a thing, I am sure there is no one formula that guarantees accuracy, though there likely is one that provides an estimation. Star and planet size, distance from one another, possibly the axial tilt of both celestial bodies, and the frequency of flares are all factors I think are appropriate to keep in mind. Essentially, what I am asking for is a way to determine the necessary strength of a planet’s magnetic field to support life using a given formula, using an Earth-like planet about 1.65 AU away from an F8 spectral type star as part of an example.
NOTE: If further information is needed, please ask. I only included what I thought was relevant, so there is a high chance I missed something important.