Building the business case for CS

Hi all-

Is anyone able to share their experience, useful resources, and/or lessons learned for drafting a business case to stand up a brand new CS function in a large, well-established organization? I'm currently working on selling CS up the chain in an org where it doesn't exist today. I'd say I have executive interest at this point, but not necessarily buy-in yet. I'm particularly interested in any thoughts on the following: 

• What was most valuable to highlight from an ROI perspective and how did you determine projected improvements by instituting a new CS function (e.g., will reduce churn by X%, will improve NPS X points, will reduce support costs by $X, etc.)? Where did you find the data to prove your hypotheses?
• Any tips on the best way to pitch a new prospective org structure with CS? For context, we currently have an AEs who handle the spectrum of new business cross/up-sells, and renewals, PMs responsible for implementation, dedicated Education team for training, and a specialized analyst group with industry experience for specific use-case assistance in the product suite and other data projects. Is there a path of least resistance you've found to at least get something stood up and evolve from there? 

I understand much of this will fall into the "it depends" category, but just looking for any thoughts and ideas on the topic. I have experience building a smaller CS team in startup environment, but busting some of the established norms and structures in a larger, well-established organization is a new challenge. 

Thanks!