Example:

I have 12/12 endless flame = -25% energy cost

I get a seal of annihilation = -10% energy cost

Does this mean FoI has a -35% energy cost?

Or a 25% + (10% of the remaining 75%) = -32.5% energy cost?

Example:

I have 12/12 endless flame = -25% energy cost

I get a seal of annihilation = -10% energy cost

Does this mean FoI has a -35% energy cost?

Or a 25% + (10% of the remaining 75%) = -32.5% energy cost?

You can easily deduct this in game now that they have fixed the skill toolbar displaying correct energy cost.