I think the easiest answer would be to pull down an oracle feed on-chain regarding what the price of ETH is in USD, and then simply compute the exchange rate each time someone comes to purchase your tokens. This way everyone is getting an accurate rate at the time of purchase and you don't have to arbitrarily set the price each morning (which might get tedious)
Chainlink has a workable oracle that you would probably be fine using. Their docs have more information you would probably find useful: