here's what you want to do.
Because it is probably still set to standard FL speeds..
and cargo size 1.
Settings -> speed and delays
Set these values (for most transports/traders)
average cruise speed: 340
average speed in trade lanes: 1900
jump gate delay: 25
jump hole delay: 15
trade lane and docking delay: 15
base reaching and docking delay: 30
then go to
settings -> limitations
cargo volume: your max, usable cargo (this would be whatever you can actually load in your hold, after factoring in turrets/other things eating up space.
In my case I've recently flown a 3500 cargo units transport.
but I can only carry 3450-ish cargo.
Enter that and set whatever limitations you might want.
It should now correctly show you profit per time.