I didn’t use an indexing jig: what i did was to a) draw a line in the router table b) put a long piece of painter’s tape on the ”board” (the piece that was already mitered but not yet glued together). On the tape i had drawn lines with the desired spacing.  Then i just routed with a sled & after each groove moved the next line to match the marking on the router table. Rinse and repeat ad infinitum. Eye-balling the line alignment was accurate enough, as  you want *some* variation. If you make it too perfect, it will no longer look hand made!