OK here's what it says on the other sites.
SV = Sand Veil, SC = Snow Cloak
Serebii - SV=20%, SC=20%
Bulbapedia - SV=25%, SC=20%
Veekun - SV=25%, SC=25%
Psypokes - SV=20%, SC=20%
Marriland - SV=20%, SC=20%
Smogon - SV=20%, SC=20%
The majority says 20% for both. Psypokes and Marriland may have copied Serebii but Smogon often does their own research to verify, so it's more than likely 20%. I will change it asap.