It all depends on the size of the free bet you are taking advantage of.
Since the free bets are mostly SNR the matched odds need to be at least 6. A £10 free bet at odds of 6 means you stand to return £50 at the bookie. The lay stake at betfair will be £8.40 but you'll need £42.02 in your betfair account to cover the return amount at the bookies if the bet wins there ( either way your net profit from the free bet would be £7.98).
The amount needed at betfair for a £25 free bet rises to over £105 at odds of 6.
The calculator will tell you all you need to know and we always advise doing the lower offers first to build up your funds slowly. A general newbie rule is to work down our list of bookies from top to bottom.