For a broadside array (radiation perpendicular to structure, such as a flat panel array), the spacing can be more then λ/2. When you approach or exceed λ spacing for a broadside array, grating lobes do appear (the same phenomenon as happens when undersampling in A/D conversion or optical gratings ).
If you use a beam steering array that can steer over a large range (so that it may become an almost end-fire array), the spacing should be below λ/2 to avoid grating lobes. In other words a broad side array can have larger spacing then an end-fire array.
Distances should be calculated using free space wavelength (assuming that your antenna is operating in air).