indeed the answer is 'suck it and see'.
Initially get your laptop(or pc) up in the loft with it, (obvious really if you want to test the bundled antenna)
Then, long term I would think about getting a 'USB Extender' (don't pay any more than £20) this will 'convert' USB to CAT5 network cable(mine is 25m from Office room to loft).
Doing it this way will ensure your losses are kept to a minimum by having as short a length of coax as possible.