The spacing between baffles is heavily dependant on the application of the heat exchanger. Some key things I picked up: When the allowable pressure drop is small, the baffle spacing and cut size should be large. The window flow velocity can be larger or smaller than for cross flow depending on the spacing and cut. Unequal baffle spacing requires a correction factor.
The most common baffle shape is a single segment. In order to ensure that adjacent baffle overlap is at least one full tube row, the segment must be less than half the diameter. Usually the maximum cut size will be used as this decreases pressure drop.
Check out the references below or have a look at Wolverine: Engineering Data Handbook III for an in-depth look into single-phase shell-side flows and heat transfer.