you can mix any flavor with any flavor ... it's not like the 'doubles' will cancel out! jk -
start with say 2% of each .. keep in mind that ry4 needs to steep ... i'd give it a day at least .. 3 is better and I usually give RY4 a week personally.
or you could start with RY4 at 4% and DCC @ say 2% if that is more of a profile you are looking for.
With TFA .. and just using two flavors I would say anywhere from 4% to 8% each, altho some like 10 or 12% of TFA.
All this is dependent on the individuals taste so really hard to say. One general rule of thumb for flavoring diy is the total % of flavors should be around 15% max - but I have seen mixes that are higher.
Hope this helps. Welcome to VU