One of the things about independent schools having charitable status is that all of the teaching, board and premises costs are classed as "expenditure on charitable activities"
So, the teaching of pupils paying full fees is still a charitable activity.
Because it is a registered charity, all of Eton's annual expenditure on teaching, board and premises (which was £74million for 2020/21) qualifies as "charitable activities"; this despite financial assistance only representing about 13% of its fee income.
In terms of pupil numbers, 260 pupils received some level of financial assistance, which is about 19% of their pupils but of those 260 only 90 pupils (which is less than 7%) have their fees fully paid.
Eton doesn't publish information on qualifying income levels for financial assistance.
Eton actually made a loss on its charitable activities but it has £450million in endowments and so on, and it made a £90million gain on its investments, so it now has total assets of over half a £billion.
Brighton College (recently lauded for giving 20 funded places to Ukranian students) has a similar level of income but level of financial assistance is under 8.5% of its fee income.
Brighton College suggests that you can earn up to around £75,000 and still qualify for financial assistance - day pupils only. (Presumably, £75k is what Brighton College refers to as a "moderate income")
They do offer 20 fully funded day places for sixth form.
However to put that into perspective, 20 free places is £188k and Brighton College made a £7m profit surplus in 2020/21. And because it is a registered charity, it is exempt from corporation tax.
It also made a further £2m gain on its investments.