I’ve seen many books etc suggest the earths magnetic field is required, but I haven’t found direct evidence for it doing anything beyond protecting the ozone layer.
"Amount of material" isn't what's relevant. It's closer to "number of atomic nuclei."
A certain mass of air is less shielding then the same mass of water. By number density, air is mostly nitrogen atoms, whereas water is mostly hydrogen atoms. Overall this means that per kilogram, water contains 2.4x as many atomic nuclei as air.
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Of course there are bigger problems with Venus cloud cities. At the 50 km height where the pressure is 1 atmosphere, the temperature is 75 °C (167 °F). At the 55 km altitude where the temperature is 27 °C (81 °F), the pressure is 0.5 atmospheres.[0]
As a bonus, both these altitudes lie deep within the layer of sulfuric acid clouds (50-80 km).
https://sciencedemonstrations.fas.harvard.edu/presentations/...
Absorbing materials and their alpha particle penetration depths.
5.5 MeV alphas: AIR(STP) 3.7 cm
2.3 MeV Beta: air 8.8 m
Solar wind is even less energetic.
Edit: “Auroral emissions typically occur at altitudes of about 100 km (60 miles); however, they may occur anywhere between 80 and 250 km (about 50 to 155 miles) above Earth's surface.” it really doesn’t take much atmosphere to stop it.
Charged particles from the Sun (SEPs) aren't what determine the design envelope for radiation shielding. Your overall dose will almost entirely come from galactic cosmic radiation (GCR) at much higher energy levels, which is significantly harder to shield against.
Those particles are what ultimately determine your shielding thickness requirements. That's true whether you're on Venus, or Mars, or a space colony.
NASA’s direct comparisons between New Mexico (55) and Antarctica (160) only showing a relatively modest decrease from earths magnetic field when measured by high altitude balloons, but again very little GCR reaches the surface.
To be clear a very small fraction of GCR is extreme high energy particles which penetrate just fine.
>again very little GCR reaches the surface
On Venus there would be less shielding than on the surface of the Earth.
The minimum acceptable altitude is ultimately limited by temperature, not pressure or radiation.