"For a mage with a 10/48/3 fire build vs a target with 5/5 Fire Vulnerability the effective damage increase is 24.63%"

I don't understand this calculation. If one scorch increases fire damage by 3%, then 5 scroches will increase it by 1.03^5%, i.e. 15.9%. How did you found 24.63% ?

Cheers, Ludovic