If you put £850 in for the next two months, you'll get £212.50 as a top up each month, but then if you need to pay nursery £1,062.50 the month after that you can just put in £987.50 and you'll get a £75 top up. £212.50 + £212.50 + £75 = £500 so that's your £500 for the next three months.
Or, you could even it all out by working out how much you need to pay to nursery each three month period, multiply by three, subtract £500 and then divide that by three and that is how much you need to pay in each month. So say your nursery fees are £1,000 per month - that's £3,000 for the three months but you will get a £500 top up towards that. So you need to pay in £2,500 over three months so that is £834 per month.
Month one you will pay in £834, it will be topped up by £208.50 to £1,042.50, you pay nursery £1,000 and £42.50 will be left in your account.
Month two will be the same so pay in £834, topped up by £208.50 to £1,042.50, you pay nursery £1,000 and £85 will be left in your account.In month three you would pay in £834, you only get £83 top up because you've reached the £500 limit, but you've got the £85 left from the other two months so you have £834 + £83 + £85 in your account. That's £1,002 so enough to pay your £1,000 for that month.