If I were a never smoker and had asthma, there's no way I'd be vaping period, let alone adding nic. Are you concerned about your wheeze? That would be telling me something is wrong, and I'd be more likely to put the vape away than try to tinker with my nic levels. Is there a reason you want nicotine at all? This is all making very little sense to me. If you have asthma, it's really not ideal to be inhaling anything into your lungs. The wheeze MAY be from the nic, or just vaping generally catching up to you. Also, vaping nic carries risks (though they appear lower than from smoking) but it's still a risk nevertheless. Why the desire to add nic at all, if it's not something you've become dependent on (yet).
I'd really see your doctor, and put the vape down for a bit, and tell him of your "plans" to go to 3 nic instead of 6. Ask him about vaping generally, for asthma, and if he feels it is a good idea. Really, it's not. It is my *belief* that vaping is less harmful than smoking, but I'd never recommend that a never-smoker *asthmatic* take up the practice.
With that said, I guess you are going to do what you are going to do. Try 3 nic and see if it's better. Though if you are getting no wheezing with 0 nic, why not just do *that*.
Anna