It partly depends on whether it's for you attacking, or you getting hit.

For attacking, 10% is reasonably good. It will proc often enough to be a regular occurrence — probably once every two or three mobs. 8% chances will be OK, and 5% chances will be kinda debatable (the effect better be pretty special).

For defending, you'll want a higher percent because you have to depend on monsters attacking, and individually they're relatively slow. Even Thunderbug's 50% chance is kinda weak against a single enemy, though it is fantastic against swarms. You'll need to look at exactly what sort of effect is being done.

So, you can't change it, but in general the percentages are "good enough". That doesn't always make the set bonus worthwhile, though.

Ashen Grip, for example, is horrible, though more due to the damage and proc conditions than the proc chance. It has the same proc chance as Red Mountain (which is generally pretty good), but does 1/8 the damage, and requires melee damage rather than any direct weapon damage to trigger it. So it's useless when using a bow, and mob placement means that the small AOE isn't going to be hitting multiple targets, and even if you did hit multiple targets, you still aren't going to do as much damage as Red Mountain (which also doesn't have to worry about mob positioning).