in my view, generally speaking, below are some principles to judge whether a witness feed price well enough.
1. feed price should reflect the price at which the conversion can be done between BTS and the pegged fiat.
2. feed price should be updated frequently enough to reflect the real market price.
3. for smartcoin with enough market depth in trading pair with BTS in DEX, such as bitCNY, feed price should not conflict with the modified DEX price, which means the DEX price plus the smartcoin premium/discount.