Increasing the Area for Epic Spells

[Otterscrubber]

When making an epic spell it says you can increase the area of effect by 100% for +2 DC. If you do this multiple times is that a linear progression, i.e. 4 increases on a fireball (or fire effect using the energy seed with a starting radius of 10ft.) makes it 50ft radius, or exponential so that 4 increases makes it a 160 ft radius?

 

[Kal Skid]

When making an epic spell it says you can increase the area of effect by 100% for +2 DC. If you do this multiple times is that a linear progression, i.e. 4 increases on a fireball (or fire effect using the energy seed with a starting radius of 10ft.) makes it 50ft radius, or exponential so that 4 increases makes it a 160 ft radius?

As with all multiplication in D&D, apply it to the base number. It would have a 50 ft radius.