I was worried about the metal shrinking, but I only had to increase the size of my prints by 1.6% to get the desired ring size. eg: The PLA ring had an inner diameter of 16.8mm and the casting had a diameter of 16.5mm.
Thanks for the info about the lack of tarnish. I'm glad I won't have to do much maintenance.
Since you cast this a few times, I have to ask: did you determine the shrink ratio empirically or were you able to look it up? This is a super cool project, and I'm impressed that you got it done before the wedding!
I looked around online and answers ranged from no shrinkage to 6%. For my first attempt I split the difference (3%), then measured the actual diameter with calipers. Also I tended to err on the side of making the ring too small, as it’s pretty easy to remove material from the inside of the ring.
Thanks for the info about the lack of tarnish. I'm glad I won't have to do much maintenance.