I see you are already try to be smart citing mathematics. Of course it is a mathematical operation that seems to make sense.

But then we have to take reality into account. Which means arbitrage will keep the e.g. BTCXMR pair according to their uniquie BTCUSD and XMRUSD exchange rate.

There is no XMRBTC. Only the proxy vs USD pairs. Simply because the reserve UoA is what literally everything gets meassured against.

Take it away and you'll see a totally different pricing.

I guess nostr:npub1lxzaxzge0jq9u9cecucctdt5lslwgp7hcxmp2l0wn8r2ecjenwasu6svxa can explain it better than I do.