How to calculate nutplate spacing method

6-8-10D, like standard fastener spacing.

if you want to "fuss" it, you could ...
1) calculate the net area, or
2) align the a/nuts with the axial load, or
3) put the a/nuts on a separate nut plate (so the a/nut rivets don't go through the primary structure) and attach the nut plate with a minimum number of rivets.

another day in paradise, or is paradise one day closer ?

Is Your rocket intended for 'one-time' or 'multiple' use.

Are You treating this like maintenance joint/panel and/or a true structural splice joint?

One row of fasteners or multiple rows of fasteners?

Are Your skins and joints designed to minimum thicknesses and mass? Or are you willing to add weight as a sacrifice for the ability to separate sections for various purposes and maintain durability flight-after-flight?

Individual 1-lug nutplate would have to used, oriented axially, to minimize installation problems with a round-body-contour... and ensure that installation rivets NEVER align adjacent to each-other [serious stress concentration].

Have You considered Gang-Channel Nutplates... or 'mounting single nutplates-on sheet-metal strip', roll-formed to precise inner/outer body contours [tight/precise fit]?

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]