Hi mate,
Best drill to do to find your optimum run up is to go out into the outfield or just some flat land somewhere mark a starting spot, shut your eyes, mark a spot to start at, this is where you'll start the run up from then just begin your run up and bowl when you feel comfortable (dont actually bowl the ball, but its good to have one in your hand) Have someone else with you to mark the spot where you landed.
Repeat that about 4-5 times, you'll start to see some consistency in where you are landing. Take almost an average from those 4 or 5 spots where you landed, mark them out and try bowling off that run up length in the nets.
Key as well is to not start your run up with a real sprint or lots of pace, first 2-4 steps for a fast bowler in their run up should just be a very gentle jog warming into the run up just making sure you hit the crease with momentum.
Good luck!