The effective Spell Power granted by this trinket is calculated as a function of cast-time. Essentially, to get the most out of this trinket, you want shorter cast times.
The formula for calculating the amount of effective +Spell Power granted by this item is -
E = 590 * 10 / (45 + (t / 0.10)))
Where...
E = effective +Spell Power
t = cast time
The following Java Program can be used to obtaina table of effective +Spell Power for different casting speeds: