Can I get a bit of insight? I've been offered a promotion with the salary increase negotiable between 8% - 12%, obviously I'll ask for the higher end

Just not sure how. First time manager as part of the role, absorbing more responsibilities without necessarily losing my current ones, we are a small team so I can probably redistribute some of what I'm doing but it will still be more work. I'd like that recognised financially.
Negotiation is, ultimately, a game of chicken. Are you willing to lose the promotion more than they're willing to lose you for the role? If you know that you're the best candidate and they
really really want you to take the promotion then you can walk in with confidence and say, "I'll take the promotion if the salary is 12% otherwise I'm not taking it". Conversely, if you are very lucky to have been offered the promotion and you know that they'd rather pick someone else, playing chicken is a bad idea!
That said, I have some more specific thoughts about a salary range. The high of a salary range doesn't necessarily reflect acceptable additional compensation for any additional expectations. If you ask for the maximum of 12% you are leaving yourself no room for growth and saying from the start, "I'm going to deliver the maximum value you can expect" which can be a lot of pressure. Conversely, if you ask for 8%, you're "cheap" and the expectations are lower, because you didn't have to promise the world to try and justify the extra 4%. The 4% difference in salary could come with 4%, 8%, 12% maybe even 24% higher expectations!
The approach I usually take is to try and get the best of both worlds: I accept the lower end of the range but make it conditional on a review in a few months. I'd say something like "I'm excited for the role, I'm happy to accept the 8% raise if we schedule a review in 3 months based on my performance" which puts you in a good, strong position:
1. If you get off to a bad start for whatever reason (not necessarily your fault, maybe you're not set up for success by the company) then you're not under pressure to perform as best possible, you've got time to settle into the role (very important for your own wellbeing!)
2. You're giving yourself time to learn the role so that when you do enter negotiations in a few months, you can walk in with a bunch of well earned confidence and knowledge, you can tell them exactly what you've done to succeed, rather than try and guess what you
might do in future
3. The company sees you as a team player, not someone just trying to milk the maximum salary (of course, you
are trying to get the maximum salary but we all have to pretend work isn't about the money)
4. A raise from 8% to 12% once you've demonstrated you're good in the role is much easier to justify when compared to being unproven (plus, there's an implicit agreement that you would get the raise)
5. Losing you would be bad for the business because they've invested time and money into you in the role, replacing you would put them back at square one
6. You're now more experienced with more responsibility, you are now a better candidate for other companies, you now have more job opportunities, you are in a stronger position in the game of chicken
You could even, depending on your confidence, ask for
more than 12% at the review. If you really excel at the role and you've started out on 12% it may take a year or more before you have the opportunity to have your salary reviewed, whereas, if you start out with 8% and an agreement to review in 3 months, and you really excel, you may be in such a strong position you can say "actually, I want 20% instead of the original 12%".
(If you pursue this strategy, you need to be proactive in making the review happen, you can't expect them to come to you. The least awkward way is to immediately add the meeting to the calendar, something like, "3 month check in" for 3 months after your first day in the new role, and in the notes, add a reference to the discussion.)
All that aside,
your relationship with whomever you're negotiating with matters, as does the size of the company. If it's a big company with lots of money, you might find that the person you're negotiating with
wants to give you the maximum of 12% because it has no impact on them and according to policy they need to "negotiate", whereas if it's a small company and you're negotiating with someone who is accountable for a team staff budget, the 12% might be something they have to wrestle with, it might be 8% for you + 4% to other employees vs. 12% just for you.
And I agree with the other response, 12% is taking the michael. For some people, they feel a loyalty to their company and their team and would struggle to be ruthless, in which case, a promotion to a manager position without a reasonable raise might not be worth it, whereas if you're willing to be ruthless, even with a negligible salary increase, it can be a big opportunity. As a team member, you're much more replaceable than a team leader.
Oh and finally, if a negotiation goes in a direction that you weren't expecting, if you don't feel like you can come up with an answer you're confident about in the moment, you can always say that you'll follow up later that day, even via email if you find you are more confident when writing than speaking. A salary negotiation rarely needs an
instant answer. If you aren't confident in an answer, you can't walk it back, and you'll spend a lifetime ruminating on it. A polite "I need a couple of hours to think about it, let me get back to you by the end of the day" reflects well because the ability to recognise when you need some time to think is a good sign for a manager, understanding which decisions need to be made quickly vs. deserve some time is a very
very useful skill in business.