I can understand about 60% of what they say on TED talk.
I usually catch about 60% of the conversations on TED talk.
この場合、次のような言い方ができますよ。
ーI can understand about 60% of what they say on TED talk.
「TED talk で何を言っているか6割くらい理解できる」
ーI usually catch about 60% of the conversations on TED talk.
「通常TED talk の会話を6割くらい聞き取れる」
ご参考まで!