Measure Twice, Cut Once

...... sez my mother who is a trained dressmaker and upholsterer.

She decided to make loose covers for my couch in the sitting-room.  She said the couch needed smartening up.

(I seriously think she was born with a measuring tape around her neck)

Her father (who was a French polisher) told her before she started her apprenticeship - 'Remember Maureen, measure twice, cut once')

..... meaning you better be damn sure of your measurements before you get the scissors out.

or as my maths teacher used to say 'Think before you ink'
(its the same thing really)