HR is treated differently than Pace (outcome) or Power (output). We take the peak 20 of a 30-minute test because we recognize that HR is a lagging indicator and needs time to “catch up” to the intensity. That typically happens in about 10 minutes at LT pace. The 95% rule on the 20 minute test just works for Pace and Power.
The 20-minute test really does not work for HR, you really need 30 minutes for HR. Or, if you have an idea already of your LT Pace or Power, then you can hit that Pace or Power and wait for HR to plateau to establish LTHR.
David