The spreadsheet you're looking at shows the point of time when the cast should start to have that specific sp/second (which is why it requires haste and all)
I have
a spreadsheet for the time of when the cast should end but its kinda out of date as the other one is more useful.
(known) discrepancies have been fixed.
The actual transition between wrath > Starfire and vise versa isnt at point 0. Just because the two spells arent identical on sp/second basis, then theres the difference in cast times which also affect the outcome due to eclipse bar moving on its won. Euphoria takes the point further away from 0 which is why people are having hard time using that talent.