210 - .65 * age is recommended MaxHr by ACSM.
For you, 210 - 18 = 192.
192 - 19 = 173, so that is about 90% for you.
If an athlete, and resting HR is, say 60, then
heart rate reserve is 192 - 60 = 132. 13 points per 10%.
Then 90% is 192 - 13 = 179; 192 - 26 = 166 = 80%.
85% of max HR for an athlete is a very strong training point. At 28, I could take mine to 205.