Combined effect of masteries and synergies

[Brista]

Fire bolt (20) + Fire ball (20) + Fire mastery (20)
expected -> 60 * (1 + 3,2 + 1,63) = 349,8
screen  ->                        668 (doh!)

Try this formula instead:

-> 60 * (1 + 3,2) * (1+1,63) = 662.8 (close enough)

In other words the mastery is multiplicative with the synergy bonus