Ahh, trying to make sense of the healing rules is really going to make me recover some S damage before I can make an intelligent reply. Ok, so you need to heal the stun damage before you can heal your physical damage, but you've done a lot of the P type healing overnight, etc.Honestly, I don't think the headache from casting would do much to offset the good work you've done with your gunshot wound, so, as long as you can spend most of today continuing to rest -- and don't get yourself in a shootout at Junior's, I'll say the 1S you took will take 1 die away from your previous healing test.
Moving forward, I do see casting as having a negative impact on healing, so I'm going to think about how that will work. I really don't see a low-level preparation interfering much, but I don't want to get into a situation where a character is "resting" but doing all sorts of casting to continue with legwork, etc.