Threshold HR from 20 Min FTP test

Is there anyway to estimate Threshold Heart Rate from a 20 min FTP test? Most estimates say to take last 20 minutes of a 30 minute effort. I don’t have a ton of HR data. Primarily power data, and I’m not racing currently. Only reason I want it is to quantify load when mountain biking. For my last 2 ftp test average hr was 176 and 177.