Gathering one’s adding a few for convenience sake (8,2)

I believe the answer is:

rounding up

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'gathering' is the definition.
(rounding up is a kind of gathering)

'one's adding a few for convenience sake' is the wordplay.
I cannot really understand how this works, but
'few' could be 'i' and 'i' is found within the answer.
'one' could be 'un' (I've seen this in other clues) and 'un' is located in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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