You have to define a lot of requirements before select the inter-elements spaicng, at least:
- peak gain
- beamwidth (in both planes)
- radiator element size and coupling
- bandwidth
the grating lobes are not a concern if you don't have to scan the beam away the boresight direction, unless you have an inter-element spacing > 0.9/1.0 Lambda.
Typically the useful range is between 0.6-0.75 Lambda.
But without the above listed information you question is not complete !