-Mana cost and skills cost 30% of life instead of mana

When using the non-channeling skills have -X to total mana cost and the new life mastery for skills cost life instead of 30% of Mana cost the numbers don't add up.

Without mastery my skill costs 29 mana.
With 2 pieces(-7 and a -6) its 16 mana, which is working properly

With mastery its 20 mana and 10 life
-6 takes it to 16 mana and 10 life
-13 takes it to 11 mana an 10 life

So it seems the mods are working at 70% effectiveness which does make sense with the mastery but if its before the calculation it should also be reducing the life cost of the skill.

That mastery does not care for reductions. It takes its value from unmodified cost.