there are two ways to measure a square bay window

Bay 1:
the side blinds can be measured as normal, then the centre blinds will be butted in between the two side blinds (L+R)

Bay 2:
the centre/main blind can be as normal, then the side blinds can be butted against the front of the main blind

with bay 1 a deduction will need to be made on the centre blind to allow the side blinds to sit comfortably against the wall. a deduction equal to the projection of two brackets will give the new recess size.

with bay 2 a deduction will need to be made on both the side blinds to allow the centre/main blind to sit accross the expanse of the window, wal to wall. so the centre blind is a standard recess size. the two side blinds will need a deduction equal to the projection of each bracket.