The breakeven point for a labs goodybag is the point at which it becomes cheaper to use the labs goodybag than it does to use pure PAYG credit.
That happens when
£5 + mins x 2p <= mins x 8p
(It gets complicated if you add texts as well).
That works out at 84 minutes.
£5 + 84 x 2p is £5 + 168p which is £6.68. 84 x 8p is 672p which is £6.72.
£5 + 83 x 2p is £5 + 166p which is £6.66. 83 x 8p is 664p which is £6.64.
So the true breakeven point is somewhere between 83 and 84 minutes; once you reach 84 minutes you're spending less money than on pure PAYG and will continue to do so however many minutes of calls you make!