Deterministic chance means creating an event set for the attribute which checks if the current event is enabled or not.
Example: 60% chance to cast Lightning Level 5 on striking.
Instead of each strike triggering a hit, 6 of 10 strikes(fairly distributed (1=cast) e.g. 01101 0 1011 ) trigger the cast.

Smaller chances (1 in X) keep a counter that decrements toward event occuring.
Example2: 1 in 200 chance to Cast Firestorm Level 20 on attack,
counter is set to 200 and decrements each attack.

Fractional case can round up to integers:
Example3: 30.3% chance to dodge melee attack
30 of 100 events(melee attack) trigger dodge.

For user-friendliness such counters/event sets can be displayed as glowing items(ready to cast) or specific graphic above character.