220- your age will not work. 220 minus age is a wild guess at your MAX heart rate. The average HR from a 20 min TT is an estimate of your threshold heart rate not max.
You will be more accurate eyeballing your heart rate during your TT and taking your best guess of your average.
Best of all buy a new HRM. Browse Amazon.com. Heart rate monitors that record average heart rate start at $23
What plan are you asking about for your zone 5 question? Whether it is a max effort depends on interval duration.
Lynda