an idea

The quote offered is sourced from a prospective customer/observer rather than from a professional source. Nevertheless, am finding the term selected precise and acceptable.
'Clearance' usually applies to the interval between the appliance considered and the proximity to its immediate environment (wall, ceiling, other equipment unit or control post) and would be found in contexts dealing with safety or installation condition specs for proper functioning.
'Spacing' fits better the context of a design description as is the case here, IMHO.
Does 'interval' convey the notion of recurrence between portholes better than 'spacing'? Seems to me that this take is derived, in both wordings, from the plurality of the objects involved, rather. But both would do, of course. 'Spacing' is informal and flows better when in combination as offered above . 'interval' sounds more more exact and technical but is less flexible to combine in a natural sounding way. So, Your choice.