£10 top up and a triple credit means £20 extra credit on top meaning you have 6 days worth of credit based on 60p per megabit. I would advise you top up £20 meaning you have 12 days worth of credit in your phone.
It help if you buy a 1gb data bundle for £7.50 and that can last quite a few days before you use your free £20 and then the paid for top up.
Make sure you top up plenty of credit to make sure you don't run out of credit as your top-up balance runs down very quickly. It's £5.00 per day based on 60p per megabit, so that is just 8 over megabit per day you will use. Keep an eye on it to avoid getting caught out.
Make sure you select a free 150 minutes call bundle before the month starts.
It is likely to cost you a lot of money with Tesco mobile it's not a joke. £10 or £20 per month top up will not be anywhere near enough to cover a full month's usage with tesco mobile.