The Sweltering mod found on Mage idols states this:
x% Increased Chance to Ignite with Fire Skills
(On holding ALT) Adds to your chance to ignite an enemy when you hit it. Ignites deal fire damage over time. An enemy can have multiple ignites at once.
For comparison, something like Increased Critical Strike chance has this description:
x% Increased Critical Strike Chance
(On holding ALT) Increases your chance to deal a critical strike. Your base crit chance is 5% so 100% increased crit chance would give you 10% crit chance if there are no other modifiers.
Given that it’s a flat addition to ignite chance and does not function like an increase, I feel that the affix should be changed to “+x% Chance to Ignite with Fire skills” to avoid confusion.