Sounds like you're getting some good training in, great! Don't worry about a computer, it won't help you stay on a wheel smoothly. When holding a wheel, especially in 40mph+ sprints, I always hear that radio-noise from Star Wars, "STAY ON TARGET!" While you don't want to stare at the wheel, you want to pay attention to the gap. As others said, look at the rider's back and hips so you can see the entire pack ahead of you and the road beyond them. Your binocular vision is what tells you the distance, you can maintain close-spacing without even looking down at the wheel. Get used to judging distance to the rider's hips with your eyes.

Then be observant and responsive. A 2-foot gap is fine for drafting, but before it develops into a 5-foot gap, it had to go to 2.5ft, then 3.0ft, then 3.5ft, then 4.0ft, then 4.5ft... it doesn't happen immediately. So pay attention and when you see the gap increase by 0.5ft, add a little more spin to your legs. Then when the gap narrows to 2.0ft, soft-pedal so that by the time you're perfectly synchronized with the speed of teh rider ahead, you'll be 1.5ft... Then when you drift to 2.5ft, add some more power... then soft-pedal. Continual feedback and adjustment. Also helps to monitor the guys and road ahead of the guy that's direclty ahead of you, because they will affect how the guy directly ahead will respond and you want to respond before he does...

With enough practice, you can modulate your power-output to be exactly as needed to maintain constant speed and spacing with the rider ahead. Then it's safe and easy to draft just 1ft behind them... no yo-yo effect. Keep up the good work!