On early cranksets like the RS2, Cooks bros, and bullseyes the spider was flat so you had to use spacers and long bolts to fit an inner ring it was the lenght of those spacers I am trying to find out, but as the inner is only really for show I have used Shimano bolts and spacers and some nylon washers which handily seem to have spaced it out pretty well...
Thanks. Didn't know that. That probably explains the issue I had. I'm guessing the previous user got it wrong & didn't notice because they didn't use the inner ring or running a thinner chain. I added a shim to the spacer that was there. I don't remember the measurements now but glad I didn't have to go wider as the inner ring is now very close to the chain stay.