Fingerprint system here too. We can top up in units of £10 so I put £50 on and hope it will last about 4 weeks.
DS has packed lunch one day a week and school lunch the other 4. Food is available before school (Cooked breakfast, croissants etc) and at break (sandwiches, cakes, cooked breakfast, croissants etc) at lunchtime (full range of hot and cold meals and sandwiches) and after school (snacks).
We manage spending by saying DS can have 1 main meal per day. He is allowed 1 pudding and 1 snack per week and no drinks. He also takes a carton drink and chocolate biscuit or fruit for a morning break snack.....so most days is just buying a main meal, which is in the region of £3. I can see what he has had electronically and there haven't been any real issues because he's had clear boundaries. If he's forgotton his break time snack, he is allowed to buy something but knows this shouldn't happen much.
I remember lots of parents at the start of Yr7 saying they topped up £50 and it was gone in a week or so. With the fingerprint system kids aren't always aware of what they are spending or prices. They get carried away with the choice and especially buying stuff beyond their main meal....so expensive drinks or extra snacks at break. Easy to spend £8-10 in a day if you're not excited by all the options.
Best way to manage it is to clearly set out what they are and aren't allowed to have and carefully checking in the early days.
I also like the ideas of having a monthly limit and after that they have to have packed lunches, or they mostly have packed lunches with a small allowance for the odd snack.