I need 248 fps with the 44cal do-it mold lead balls to reach 15 ft/lbs. Right?
The only way you can be certain of ball weight is to weigh them. Your alloy can vary weight by quite a bit. I have two Do-It molds and my current batch of .44 balls (actual diameter .429) cast mostly from wheel weights weigh 116 grains. To make it easy, here is an an Excel spreadsheet that will do the math for you. It gave me 242 fps with my balls for 15.09 lb/ft.
Do I need a special scale?
Yes, but they are cheap. I have an AWS AC-Pro 200, accurate to ,01 gram. It cost me $20.00. I bought a calibration kit with it and shipping was free.