Forge Bonuses

[SurvivalistMerc]

I am a bit confused on this and would appreciate it if a more experienced player could clarify which bonuses are cumulative with which.

Specifically...

ULM Smith/Inquisitor: 25% forge bonus
Dwarven Hammer: 25% forge bonus

(I know these are additive and that it takes 50% the normal cost to craft an item, rounded down. That rounding down often makes for a 60% reduction in item cost, which I really like.)

Cyclops Hammer: I'm assuming that this would be additive in a manner similar to the Dwarven hammer.

Spell: Forge of the Ancients: This supposedly gives a 50% reduction in gem cost.

Now...is this 50% achieved by multiplying the cost previous to it by 0.5? Or is it achieved by adding 50 to the total percentage reduction? If the latter, Ulm smiths would be able to forge gem-free with dwarven hammers once this spell has become active.