Spell Buff is a multiplier applied to your spell's base value. The easiest way to see this in the healing miracles, where the base value is unaffected by other variables.
Consider the miracle Sorcerer's Staff, which has a Spell Buff of 60.
To apply your Spell Buff, divide the value by 100 and then multiply the base value of Heal Aid with your result.
For our example, this would be (60 / 100) * 150, resulting in 90.
However, your actual Spell Buff value is not just the Spell Buff listed for your weapon. It is modified by the amount of INT/FTH you have (depending on the tool) and by the level of reinforcement the tool has.