I have to agree with the poster above who said "let your ears decide" and that there is no standard recommended percentage.
Streaming transports vary wildly in pricing and features, and are one of those areas where spending more does not necessarily produce audible benefits. Sometimes it just gets you more features, or a big fancy enclosure, or a recognizable brand name. If those aspects are useful go for it but for some people they aren't necessary.
For example I hear nothing but raves about the Stack Audio Link which is relatively affordable and has very limited features. But if it does what you want or need it to do, then it just might outperform much more expensive alternatives.