If a spellcaster can wear light armor without suffering ASF, how does mithral medium armor work?

Question

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?

Somatic drawback

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.

Mithral

Most mithral armors are one category lighter than normal for purposes of movement and other limitations.

and

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?

Answer 1

Mithral makes medium armor count as light for all purposes except proficiency. So yes, if you ignore arcane spell failure in light armor, you ignore it in otherwise-medium armors made from mithral. That means the ASF reduction effect of mithral is irrelevant to you, as whatever the ASF is doesn’t apply to you no matter what its value is.

Most classes that ignore arcane spell failure in light armor only have proficiency with light armor, however, making the use of a mithral medium armor not worth it—the improved AC isn’t worth a feat (particularly when considering there are other drawbacks), and multiclassing is punitive for most spellcasters. If you were already multiclassing, though, or for some reason your class that ignores ASF in light armor comes with medium armor proficiency, it could be worth it.