In battle 20%
Sand Veil increases the evasion of a Pokémon by 20% in a sandstorm and if on a Pokémon whose type does not already, grants immunity to damage taken from sandstorms.
In battle - bulbapedia 20%
Raises the Pokémon’s evasion during a sandstorm by one level - serbii 25%
During a sandstorm, this Pokémon has 1.25× its evasion, and it does not take sandstorm damage regardless of type
The evasion bonus does not count as a stat modifier.
Overworld: If the lead Pokémon has this ability, the wild encounter rate is halved in a sandstorm - veekun -25%
Boosts the Pokémon's evasion in a sandstorm. This Pokemon's Evasion rises one level during Sandstorm.
http://pokemondb.net/ability/sand-veil - our very own site :D 20%
http://pokemon.neoseeker.com/wiki/Sand%20Veil - pokemon wiki 20%
im not entirely sure but i am going to go for 20% because of all the websites ive looked on appart from serbii have by the looks of it not that reliable info
to be honast 5% doesnt make that much differance
anyway i have just looked up similar ability snow cloak and that does 20% evasion in hailstorm
hope this helped