Below what percent concentration would that be equivalent to?
Around what percent concentration would that be?
The concentration of this solution would be below what percent?
完璧な直訳ではないですが、Around what percent concentration would that be?は約何%の濃度と言う答えをもらえますね。
Below what percent concentration would that be equivalent to? はより直訳的です。何%以下に相当しますかと言う意味です。
the concentration of this solution would be below what percent?は一番直訳的な表現です。通じますがちょっと硬く聞こえます。
ご参考になれば幸いです。