It is fairly easy. For a hotel or anything non flight, you are charged roughly 5.5p per mile. A flight redemption can get you 8-10p and 20p at times, so are the best use. If offered cash and miles and the cash bit equates to selling you miles for 8p or less, then take it - as long as you will use those miles for flights themselves.

Plenty of advice in this thread on flights, but basically the shorter haul the better ie Dom, AMS, Par, Bru and in Economy. The total opposite of BA Miles!