If you are interested in the math, the chance of fizzling is based on your magic skill, the minimum level of that magic skill required to cast, and the maximum level the spell will take magic skill up to at full speed. This is done for both Magery and the particular skill the spell uses. Successful casting requires success for both skills used by the spell. The chance of success for each skill is (amount of skill you have - minimum required to cast) / (max it will raise you to - minimum required to cast) * 100%.

For example, Empower requires 25 Thaumaturgy and will raise it at full speed to 50, and 35 Magery and will raise it at full speed to 60. So, if I have 31 Thaumaturgy and 50 Magery, the chance for the spell to succeed on Thaumaturgy is (31 - 25) / (50 - 25) * 100% = 24%. The chance to succeed on Magery is (50 - 35) / (60 - 35) * 100% = 60%. The chance for the spell itself to succeed is then (24% * 60%) / 100% = 14.4%.