You start by getting your saddle position correct. KOPS is a good starting point, but from there tune the fore-and-aft position for balance: while riding down the road in a low position, you should be able to briefly take both hands off the bars without sliding forward on the saddle. If you slide forward, move the saddle back. If your hands feel too light, move it forward. Most people need a setback seatpost to achieve proper balance. KOPS is merely a first approximation. Balance rules.

Then worry about the stem length. A simple method is to ride hands on hoods or hooks, forearms more-or-less horizontal. Your elbows should be in front of your knees by 1 to 4 cm. That will produce the normal stretch for a comfortable yet fast road position. Another way to evaluate reach is to position yourself while looking in a large mirror. With your back as straight as you can get it, your upper arms should make a 90° angle with your torso when your elbows are bent throughout the range of hood and drops positions.

If you can't achieve these positions with normal length stems and saddle rails, then your bike is probably the wrong size for you.