Unexpired Basis

Hi,

I am really struggling to work out the rule of unexpired basis risk when working out the closing future rate for both Interest/Currency Futures.

I think it looks like – (date wen funds are received/paid to the futures loan start date) / (todays date to loan term future)

Ie – if today is 1 Jan, we borrow the funds on 1 May to repay in 9 months time. If we select the June Future then the unexpired basis risk is

2 (May to end of June) / 6 (Jan to June)

Is this correct or can you explain a general rule ?

Thanks
James