As others have said .. benchmark the new role in other companies in your area or similar areas nationally.
Contact local agencies and see what you would be worth on the open market.
If you can, look at advertised roles within your company so you can get a feel for other rates of pay. You don't want to end up suggesting your role should be paid more that the department director.
Speak to a few agencies if you can so that you know whether you would be likely to get a better salary elsewhere and whether others are recruiting, this will give you a level of confidence in case you don't get what you want or will let you know of the market is tough and and you should tread carefully.
Do some business analysis of the increase in customers in the past year and show the value you bring to business or the value of additional income that your job will be supporting.
Try to stay clear of too many %s as you may well be already negotiating from an underpaid position so a 10% increase may still put you well short of the benchmarked level for your role and well short of the increase in business you will have to cover... if the business is doing 50% more turnover for the same staff costs, you deserve a cut!
Think about whether a review and further increase based on value of new business introduced would be in your favour.
Also think about how you pitch for new business and how you would present to the people you work for in your new job so that in this process you can showcase the skills you have to do your job well so they walk out of the meeting thinking, yes... this is exactly why we gave Fressia the promotion and she deserves to be paid accordingly.