400mg/ml test enanth is pretty easy to make because enanthate is soluble in oil. when i made it i used 3%ba and 8%bb for 450mg/ml. you could probably go as low as 2%ba and 9% or 10%bb. Don't worry about the kind of high bb concentration...i did a full ml of bb last week to test the pain and it only stung for a little while, nothing after that.
another thing with a higher concentration like this is it's pretty thick, mine still went through a 25g fine, but don't be surprised if you need to use a 23g.
how many total ml do you want to make? i'll show you how to figure it all out.