Setting starting elevation, especially if the start is typically from the same place, like home, does get the numbers in the ballpark you expect to see. The OP stated "-457" so obviously setting it does tell you you are not in Death Valley.

Not sure if the Wahoo Mini has barometric sensing though.

Bottom line though is it seems EVERYBODY complains about elevation gain/loss numbers with various GPS devices and the data that goes to the web based trackers.