In the most optimistic scenario (2,4)

I think it could be either:

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2,4

at best

'in the most optimistic scenario' is the definition.
(I've seen this before)

This is the entire clue.

(Other definitions for at best that I've seen before include "In the optimal situation" , "Making every allowance" , "Assuming the most favourable conditions" , "the most you can expect" , "In the most favourable interpretation" .)

I've seen this clue in The Times.

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