In regard, to your comment about posture, I agree that breathing and posture go hand and hand, one think I have noticed is if I exhale too much air my core will release and my posture will be compromised.
:-) you're very right! Everything in FS is interacting with each other in whole stroke. But most times it's extremely helpful to just focus (that's why it's named so) on/in one
single thing and get it correct (more or less) and then
try to integrate it in whole strokes. The drills are designed to help you with your special FPs... and breathing is not covered with a single FP.
According to your problem: It's not a fault to hold a rest of air before inhaling. You have to find out your individual comfortable amount to hold. You'll find out very fast, it wan't be a constant. It depends on SR, pace, practice, chosen FP and... and... Surely you'll find out, sometimes it will feel relaxing to exhale all "used air" in the last moment before inhaling, to get fresh air in your "tank"...
Play with it, stay aware and go on with your good work.