Every situation is different. I have a really good customer that creates several drops in their manufacturing process. Sometimes I have to cut their parts from these drops & sometimes from new plate. The drops of course take much longer per part to haul, handle, array parts to odd shapes, deal with sometimes slightly deteriorated plate, etc.
You have to make money or you'll find yourself needing "A BIGGER TABLE". Experience has to be your guide.
When the guy comes in carrying his own metal you can bet you need to charge more per hr per inch or however you charge. In most cases you can bet on rust, odd shape, warped because it was ran over by his tractor.
Most of my work is more production oriented & for other businesses. On a new part I usually watch my time pretty closely, I actually use a stopwatch, start it when I climb on the fork lift & stop it when I'm done with cleaning off the table. The stopwatch allows me to stop & restart the time as many times as I need if I have to take phone calls or when nature calls etc. That way you are charging your customer an honest wage on an honest amount of time spent on his job.
I most usually turn this time into a per part price so that repeat parts are already calculated.
For me there isn't a simple solution. It has took time.
If it works.....Don't fix it!