Rant: my first side gig quote was 40% too low and I almost quit over it

Six weeks ago I quoted a guy $300 to build and install some basic shelving in his garage over in Elk Grove and I finished the job in one long Saturday and barely made $90 after I paid for lumber and gas. The problem was I priced the job off what felt fair instead of tracking my actual hours and material receipts from the two small jobs I did before it, so I was basically paying myself $11 an hour without knowing it. I started logging every minute and every Home Depot run on a $2 notepad app and my next quote came in at $480 for the same scope, so my question is how do you guys break the news to a repeat customer when your rate has to jump that much?