Anyone know of a way to find out fore sure what the increase is?
Open up starfarer-core/data/hullmods/ECCMPackage.java, look at the constants.
It looks like the counter-flare chance is increased by a flat 50% (so a 15% chance would become a 65% chance, for example). I'm assuming the guidance gets a 100% boost from base; at least, that's what the (unused) tooltip implies.
Edit: Actually, the guidance is also modified by a flat amount. I'm not sure how that works, because I don't know what units guidance uses. Sorry.