It's a difficult task getting flanges measured accurately, made slightly complex by the mix of Imperial And Metric sizes. I've sorted my flanges into 5 plastic bags by size, so in a case like this I was able to go to the box and check quickly.
It needs some thought, some measuring, particularly of correctly paired lenses & flanges, comparison of the outside diameter of a lens thread compared to the inside diameter of a corresponding flange.
I've found it useful to meet others in a similar position, looking for flanges and swap when we have matches. I have a few flanges that initially appear to be the same size and thread using thread gauges, but in practice they aren't.
Ian