What minimal distance between patch antenna and plastic enclosure can be choosen?

How to choose minimal distance between patch antenna surface and plastic enclosure surface? If distance is 0mm then there will be shift in center frequency, and maybe beam pattern, some phase shifting in feeding network Moving enclosure surface farther (0.1*lambda, etc.) will give less impact. How to choose minimum distance?

By finding the distance that gives your largest acceptable frequency shift, obviously

Select the gap you need and adjust patch length to compensate for frequency shift - that's what I do. Gap can be pretty small as soon as plastic does not touch patch surface.