I am playing a spheres of power spellcaster who has taken the somatic drawback, but only once. This allows me to wear light armor much like a magus is allowed to and not suffer from arcane spell failure.
Now assuming I have taken the feat or some how acquired medium armor profiency, how would mithral medium armor work with the ability to wear light armor with no ASF?
You must gesture to cast spells—a process that requires you to have at least 1 hand unoccupied. When using magic, you cannot wear armor heavier than light without incurring a chance of arcane spell failure.
You may select this drawback twice. If taken a second time, you cannot wear any armor or use a shield without incurring a chance of arcane spell failure.
Most mithral armors are one category lighter than normal for purposes of movement and other limitations.
Spell failure chances for armors and shields made from mithral are decreased by 10%,
Light armor has between 5% and 20% ASF and Medium armor has between 20% and 30% ASF. So does this ability allow me to ignore 20% ASF and mithral allows another 10%? Or does it simply change the medium to between 10% and 20% and does not care that I can cast in light armor?