I found this a while back and found it helpful for determining the 1/4, 1/2, 3/4 effort:
"1/4 effort - easier, but still aerobic pace (not jogging), 65-70% of (Maximum minus resting heart rate). For example, with a resting of 55 and max of 195, it works out to 145 to 153 beats per minute.
1/2 effort - run at a strong aerobic, but sub tempo pace, 70-75% of (Maximum minus resting heart rate). For example 153 to 160 beats per minute.
3/4 effort - run at tempo pace/At Threshold (AT) pace, 75-85% of (Maximum minus resting heart rate). For example 160 to 174 beats per minute."
I raced a 1/2 Marathon in May and wore my HRM and determined that my average HR during that race ~ 5:20/mi pace was 178. Based of that when I started building up my base again this summer I focused on trying to keep my 1/4, 1/2, 3/4 paces around that figure. 1/4 felt slow at first, and 3/4 felt fast- but after 6 weeks of it- I'm pretty sure I can hit those paces without my HRM. Assuming I stay healthy and I gradually get more fit, I'll eventually be running at the same effort- only I'll be going faster. Thats really what I think the 1/4, 1/2, 3/4 efforts are trying to get across to people. Nobby, do I have the right idea?
Here is the discussion:
http://www.bunnhill.com/BobHodge/Special/LydiardInterpreted.htm