Only just how one measures rainfall? (2,6)

'only just' is the definition.
(phrase meaning very closely)

'one measures rainfall?' is the wordplay.
I cannot really understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
'measures' could be 'es' (e is a kind of measure) and 'es' is found in the answer.
This may be the basis of clue (or it may be nonsense).

'how' acts as a link.