final minute of ramp is a long way from 5 min power for most. If you can hold that power for 5 mins why would you crash out after 1 min holding it on the ramp. Appreciate there’s fatigue in the legs already from previous stages but you’re probably only above FTP full stop for 5 mins by that stage…
Your max 1 min on the ramp is 133% of FTP which is crazy for 5 mins. E.g. FTP of 374w expected to hold 500w for 5 mins which is inconceivable. That may be the reason for your estimation being a long way off. Probably best to test 5 min power directly and go from there. Or use your 3 min power from a TR VO2max interval and use that to gauge an approximate 5 min power.