This is certainly a very personal question. But I think a general rule could be that your family’s lifestyle will not be materially affected by the tuition payments.
I don’t like the idea of borrowing money or selling major property to fund BS education. So a 60-70k annual payment after taxes translates into 90-100k pre tax. You should be able to pay that amount from your income in addition to other expenses. Don’t forget annual fund raising. I heard some prep schools have a target amount per family.
Just my 2 cents.