For one, there are very few advantages to shooting bupe at all; and I hope you are aware that you are only doing it because you are addicted to the needle. No judgement, I've had my struggles with needle fixation myself; but you need to be aware of it in order to face it.
Because even though it has increased BA it wears off MUCH sooner so you aren't saving much by doing this, there is no rush, and unless you are waking up in WD and that first shot is relieving them a little quicker there really is zero advantage. If you disagree, don't bother defending it to me, it'll just be rationalizations you are making to yourself to satisfy the inner addict(the use of the needle, bigger is always better, etc.)
WIth that said, 2-3mg IV of bupe is a very large dose, you should never need more than 1mg of bupe IVed and could get by on much less and in fact it is going to act most like an agonist at .5mg or less. It will take some small time to adjust to a lower dose, it's not like if you've been shooting 3mg at once then .5mg is going to suddenly be stronger, but if you can get it down, teeter on the edge of WD then start using the lower amount consistently I promise you will find it more satisfying eventually. This applies to sublingual use as well.
But your first priority should be getting away from the needle, and I know how hard transitioning to sublingual use can be, my suggestion is to create a nasal sprayer with a pump nasal sprayer and some saline. If you are buying it retail and not order an empty one online it will probably come medicated, empty it out, and refill with saline. IME almost all pump sprayers dispense about 0.1mL of water per spray(at least the traditional smaller kind, I have seen larger saline pump sprayers which I haven't measured). So keep that in mind when making your solution, 10mg/10mL at 0.1mL per spray means 1 spray = 0.1mg of bupe, using this you should be able to figure out how to make it whatever strength you want with a little math.
I also suggest to make it a little strong and limit the number of sprays at first, rather than end up using 6 sprays at once, then titrate down; but if you find yourself using more sprays than intended because you keep telling yourself you need more, then you may want it more dilute, but at first I think your priority should be getting away from the needle, and not worry about how much bupe you are using.
