I can see you're new, so I'll give you my strategies when I look at a new flavor.
Firstly, percentages vary widely from person to person. With that said, Capella's is normally in the 7-15% range. For pretty much any flavor from any manufacturer, this is where I start. This is always nice because you can add more to figure out about what % is best.
Also, another thing I do when looking to
buy or work with a flavor is google it's name:
https://www.google.com/?gws_rd=ssl#q=capella+horchata ... In this case we see a few
threads talking about it, most of which are in ECF. I see one person used it at 7% in a mix with other flavors, so that's a good point to work off of.
I will say I love me some capella's. They are the best I've tried, and they are also very forgiving. I mixed one of their flavors at an absurd 25% just messing around, and I got no aftertaste. It was pointless as the flavor wasn't enhanced anymore than at 15%, but it was still nice to know it's so forgiving.