10% of RY4 works for me as well. however, I do like to air it out a bit. Otherwise, it's kind of perfumy to me. also, I like to let it steep a bit. Especially if i mix it straight.
I prefer double RY4, personally. So eventually, I would definitely recommend that you get that. It's a very unique taste, especially steep wise. I did 10% for my first batch and it was good to go immediately. However, upon steeping for a couple days, the flavor completely changed. (was still good.. just different)
As for the percentage on the flavors (caramel and vanilla) it really depends on the brand. Also, did you plan on letting it sit a bit? (steep)