Questions about buying altcoins , someone help

So I’m kinda new to alt coins and crypto in general but fast learner... one question I have now is when buying alts on exchanges like binance how do you know when you’ve made a profit? and how do you know if it’s better to buy BTC or ETH initially to trade for your alt coins. Bc I’be noticed with profits in binance it tells me what I paid it BTC but what about when BTC goes up or down how can I know without doing the math if it’s in profit or not(I’ve only used mobile site on my iPhone so maybe on the computer it shows more info that I haven’t seen yet). And when wanting to buy alts how can I tell which out of Btc or ETH is better to buy to trade for alt coins , do I go by what the % change is compared to the alt I’m trying to buy or does it not matter at all if I buy one of the 2 ans instantly trade them for alts before prices have time to change?? I would really appreciate soemone helping me out with this.