Market Cap divided by 21,000,000
Divide a coins market cap by 21,000,000 to give an idea of what the value of any currencies coin would be if it had the same amounts of coins as BTC fully mined. This includes Bitcoin.

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Frere Lineon commented
Excellent idea. In my "crypto-investment" table, I use two similar ratios that show by what factor a coin still could grow to reach BTC valuation :
¤ Potential Growth (actual) PGa = 1/BTC price * (BTC supply / coin supply)
¤ Potential Growth (total) PGt = 1/BTC price * (21 million / total coin supply)
The PGt for Litecoin for example is a mere 25 (@0.01BTC/LTC), while the PGt for FeatherCoin is still over 300 and Worldcoin could grow 3,000 times in order to reach today's BTC rates...