With marriage the starting point is to decide the sharing of capital. If
you aren't married this is usually determined by the deeds of any jointly
owned property, so the property is shared 50:50 unless it is stated
otherwise. However, if a cohabitant has the means a claim can be made under
family law against them to provide housing for a child until the child
reaches maturity.
Spouse maintenance depends upon the
shortfall between the income and living expenses of the recipient, and the
difference between the payer's disposable income once their living expenses
are deducted from their income. There is also an element of sharing.
When the sharing in capital is in favour of one spouse their
mortgage payments are less reducing their living expenses and "need" of
spouse maintenance. In this case the other spouse will have less capital
and a larger mortgage increasing their living expenses and decreasing their
disposable cash, and their ability to pay spouse maintenance.
As Mike said SM of more than 50% of income is exceedingly rare, but very
occasionally SM plus
child maintenance can amount to 60% or more.
Spouse maintenance is only paid when there is a big discrepancy in
incomes. Usually once state
benefits are taken into
account someone on average or below average income is unlikely to pay any
spouse maintenance.