Crazy C - I always adjust HRM's upwards by 10-20 BPM. According the the scales, my 75% is 139BPM. As Cassandra & Veronica have said, my perceived level of exertion is different - I feel like I'm just getting warmed up at 139BPM. I perform best at 155 - that HR feels more like 75% to me.

Your mileage may vary. I think those of us who are more fit can handle a higher heart rate. I have no scientific basis for that other than my own gut feeling.

I love gadgets, but for me, I need to make sure I'm not turning my cycling into a science project.