It depends a lot on WHICH Capella flavoring you're planning to use. Here's a spreadsheet I refer to in order to a general idea for my starting point,then work up or down to suit my taste:
As you can see, the Capella suggested percentages vary from 7% up to 15%. Mix only one 30ml bottle at a time so if it's not what you want you can change for the next try.
Edit: Hmmm. I didn't expect it to post the entire spreadsheet. Here's the link itself (broken) so you can refer to it later without having to find this post again.
https:// docs.google. com/spreadsheets /d/ 1agXdua8bEp8MV4z1mn3qX4AXCS4P0RxQXcy2lDa7ccc/pub?gid=0