There are 4 different types of worms you can use. To have guaranteed profit, you must use the small green type - smalls. The other, better worms give more xp per cast and a shorter cast time but they cost more and are meant for fishers with high xp totals and more experience on when to use them.( Fishers who are focused on gaining xp quickly and don't need profit right now.)
TO HAVE GUARANTEED PROFIT YOU MUST USE SMALLS
To figure out correct profit, you will need to average your costs over a 100 cast spread - choose your buoy to begin fishing, any buoy will do, 1x or higher.. Fish out your complete allowed casts at that buoy before moving on to the next one. For the purpose of this example we will say 45 casts from a beginner rod. Now go to your next buoy and again, fish up your complete allowed casts. That's 90 casts and is not enough to get the average needed so go to a 3rd buoy and fish up your complete allowed casts. You now have enough casts to average your total profit.
TO FIGURE OUT CORRECT PROFIT YOU MUST AVERAGE YOUR COSTS OVER A 100 CAST SPREAD
Take the cost of the worms used for the amount of casts made and subtract all worms won from fishing during this 3 buoy spread. If you didn't notice it at the time, go back and check local log history for the worms won each time you completed a cast. Now, subtract the cost of the new worms used total from the amount of lindens earned during the 3 buoy spread. There is your
YOU MUST SUBTRACT WORMS WON FROM YOUR WORM COST BEFORE FIGURING PROFIT
There you have it!