This is the aspect of Cryptocurrencies I've been trying to get my head around. What I have done when trading is to compare the dollar value of a coin to its BTC value and then calculate what the BTC value is in dollars. If I'm buying I use the one that has less value, if selling I use the one that has higher value. One of the pitfalls of doing this is the high volatility of BTC so it doesn't always work but it can save a few dollars.