If full price is £9.99 and you've got it on offer at £5.00, that's less than 50% off. If you want to say you're offering it at 50% off, you need to price it at

If full price is £9.99 and you've got it on offer at £5.00, that's less than 50% off. If you want to say you're offering it at 50% off, you need to price it at £4.99. In banking, I believe the convention is to round to the nearest penny, and round half-pennies to the nearest even penny.