The idiom 'all by the same measure' means everything follows one consistent principle.
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揆 is literary and rarely used alone. The 'estimate' sense survives in 揆度 ('to weigh up') and 揆情度理 ('to judge by reason'); the 'premier' sense survives in 阁揆 ('the cabinet head'). The everyday word for estimate is 估计.
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Literary and formal — classical prose, idioms, and political vocabulary, not everyday speech.