The new rules are clear and very specific.
The principle is this, you MUST have a proven income of minimum £18600 for spouse
(for a couple with no children) Children who do not need a UK visa are not counted at all.
The evidence needs supporting for at least the 6 months prior to application.

Just to be clear, child benefit (nor any other benefits) can be included in the £18600
But can of course be claimed by the UK national as normal.

The UKBA has no or intention to draw up a personal financial balance sheet for each couple (outgoings, credit card and other debts, mortgage, etc), but to take £18,600 (or the higher level for children) solely as a benchmark for financial requirements on the part of the couple.
Means no interest in 'affordability' only in the threshold.

Take a look here for a better understanding