The ability's success rate starts at 60% and increases by 1% for each of your levels of Invocations and points of intelligence. The amount of MP recovered is a random number between 1 and ((Invocations/4)+2); you'll only gain 1 or 2 MP per use with no Invocations skill, while an Invocations skill of 24 will net you between 1 and 8 MP. Note: The division Invocations/4 is done using the function div_rand_round which randomly adds one if the remainder is not zero.
Evoking a staff of energy allows you to channel energy in a similar manner, recovering 1 to 3 MP at a cost of 50 food per use. Its success rate equals (Evocations+11) × 2.5% (Minimum: 27.5% with 0 Evocations, Max: 95% with 27 Evocations). Although this is a slower rate of recovery, the food cost is drastically lower, making it a much more efficient method outside of emergencies.