Rough calculation (8)

I think it could be either:

Which length description are you looking for?:

estimate

'rough calculation' is the definition.
(I've seen this before)

This is all the clue.

(Other definitions for estimate that I've seen before include "Gauge - approximation" , "Make an educated guess" , "rough judgment" , "Calculate approximately" , "damage" .)

I've seen this clue in The Irish Times, The Star Tribune, The Telegraph and The Mirror.

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