Iyasenu is right.
Mana Reduction status reduces the cost by 10%. Meaning you need 90% of the cost to cast the spell.
Mana Reduction upgrades reduces the usage by xx%. Meaning you still need the full cost to begin casting the spell, but it will use the appropriate amount.
Mana Reduction with a -16% wand would require 37.8 mana to begin casting (90% of 42) and then use up 31.7 mana (84% of 37.8).
Multiplicative means it's " x (1.00 - 0.10) x (1.00 - 0.16) " and not " x (1.00 - 0.10 - 0.16) "
... or that it's (x .90 x .84) and not (x .74)
Like if you use a 50% off coupon on something that's on sale for 50%. You don't get 100% off, it ends up being 50% of 50%, or 25%.
Explain this to me. i cannot understand o -o