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