It really depends on the situation.
Going to try for PR that will take longer than 40 min. Starting conservative is good.
For doing TR catalog workouts that are threshold or lower? 360 should be just about right for a but the most extreme threshold ones.
Doing VO2? Cant really peg it that easily off of FTP anyways. It might be 120% of 350 or 117% of 360. Same difference in the end.