If I'm looking after children of different ages, how do I work out ratio's?

ie; I have 3 children, aged 2,3, and 6, and would like to start a childminding business, with the hope of extending this to a small nursery. How do I work out ratio's when I have such a variety of ages, and bearing in mind my own children's care, how many children can I 'take on'? As my husband will be at home some of the time, does he need any childcare qualifications too? I am also interested in collecting children from school; do I need special insurance, and what are the ratio's then? Please help, I am a little bit confused!!!