Assuming that 309 watts was your maximum one-minute power when following the Ramp Test protocol, then yes, this is the correct calculation
Mount Hayes -2 should be tough but manageable. That final interval at threshold should definitely require mental fortitude to power through to the end
It sounds to me that your FTP is set a little bit too low. When you completed the test, did you feel like you gave it your all? At the end of a Ramp Test, you should be sweating and gasping for air; a complete maximal effort.
If you do feel that it was a maximal effort, but your FTP is still a little too low, I’d recommend bumping your FTP by 2-3% at a time until you find a level that is “tough but manageable”.
Let me know if there is anything else I can help you with!