What Luc is saying is you don't need to convert the currencies at all when you're trading. Look at one trade-pair at a time. You buy something low, you sell it high relative to where you bought it. You either sell everything you bought or you just sell your capital and keep the free coins.

Sometimes ETH/BTC will break a base but ETH/USD won't, so trade ETH/BTC that time. Don't think about the amounts in USD, except to make sure you're within your overall risk tolerance.

Sounds like what you need to do is pick a priority: Are you trying to generate cash income, or are you trying to earn free coins to hold long term?

If you're mostly trying to earn income, then stick to USD trade pairs, and always sell your whole position at the end of the trade. You won't keep any BTC or ETH to do many alt coin trades, so you'll be limited to whatever you can trade against USD. Careful: lots of alt coins that trade against USD do so at very low volumes. That can sometimes be good for volatility, but bad for liquidity: you could get stuck with something you can't sell at the price you want because there aren't enough buyers.

If you're mostly trying to build a crypto portfolio, then start with USD trade pairs, but leave your profits in as free coin. So if you buy $150 of something, sell $150 of it when you complete the trade and leave the profits as free coins. Eventually you'll build up a stock of BTC and ETH (and DOGE, LTC, BCH, etc) all for free. Then, when you trade those against alt coins, again leave the profits in the alts, and you'll be in them for free too -- if they crash, you didn't "lose" anything, but if they go to the moon, you'll be along for the ride free of charge!

If you keep your coins but still want money to pay the rent, you'll have to wait for bull markets that will allow you to sell little bits here and there without compromising your position. Or just keep your day job.