Originally Posted by joejack951

There will be a little gap when it's fully tight even at the right size. As Operator pointed out, if the clamp was actually too small, you wouldn't even be able to close it enough to get the screw started. If it was too big, it wouldn't even begin to tighten. If the measurement seems close to 31.8mm, then get that size clamp. Or better yet, buy a braze-on derailler and 31.8mm adapter. Worst case, if you somehow got it wrong, all you need is a new adapter. Also, if you want to move the components to another bike, you'll only need a new adapter, not a whole new FD.