Ok, there are a few different options/issues here.
First, you need to know that there is a distinction between spousal maintenance and child maintenance.
It may well be that you would be entitled to spousal maintenance from your husband if he earns 3 times more than you - whether you would be entitled and how much to would depends on lots of factors (such as length of the marriage, both your incomes, your needs and outgoings etc) which a solicitor could advise you on. It is not usually based on a percentage or formula, but on assessment of needs and outgoings. It is entirely separate from child maintenance.
Child maintenance is usually paid by the non resident parent to the resident parent and is based on a % of the payer's income (15% of net income for one child, although there may be a few adjustments to reflect shared care) regardless of the payee's financial position. So if your daughter were living with your husband, in theory, you would have to pay him 15% of your net income, regardless of your incomes. BUT this is only if he applied to the CSA - if he didn't, you are free to agree something else between you as part of an overall financial package.
If there is shared care i.e. your daughter spends her weekends split between you, the CSA has a very stupid way of assessing who is the "resident parent" namely they see who has the child benefit book. If you have it, then hang onto it!
The other thing is that your ex cannot legally stop paying child maintenance and pay school fees with the money instead - the two are separate. If he is bound to pay you child maintenance (or indeed spousal maintenance) and wants to pay school fees on top, that's fine, but he can't simply stop paying one to pay the other.
HTH.