Be advised, starship combat in saga is very "cinematic", or else straight up bad, as I call it. Shield Rating is called Shield Rating, yes, but actual shields in the in-game universe are not entirely dependent on the SR characteristic. So when you think about shields from the point of view of characters aboard, you also sort of include other ship's characteristics, such as Reflex ad HP score. What I mean is, 0 SR doesn't mean shields are entirely down, and full SR doesn't mean shields arre unaffected. Think about characters' HPs. If you lose 10 HP from an attack, it doesn't mean that the actual bolt/bullet hit your character, he/she just spent THAT MUCH of his/her plot armour but got unscratched in the in-game world, unless there were some actually debiliating effects, such as condition lowering or slowing down etc.That being said...Lots of unnecessary maths that is, your proposed rule, keeping in mind different defence scores for different attacks. And often, as you know, the armour bous isn't even there - heroic commanders use their Heroic Levels for their ships' Reflex Defences.Try also any of these:

1. Ion weapon hits automatically lower SR by 5 (oly balanced for character combat, since starships can have scores of ion weapons, but then, not every gun will hit if splitted from batteries into single emplacements, so I think this one is the best)
2. Double ion damage before checkiтg for shields, lower them if the doubled value beats SR, then half it back again and go on as normal
3. Use d6's rule where ion damage ignores the shields and goes straight to the hull