The fast charge period is to an absolute point, not a relative one. It isn't that it takes 30% to put in 60% of the charge (to use the Tesla 3 example) - it's that you can charge at a higher rate UNTIL you get to 80%* charge and after that you have to drop down to the standard charge rate (which is very slow for that part of the charge anyway). So using fast-charging you can only ever get to 80% charge, That means that the 40 mins at economy cruise becomes only 32 minutes, and even with only a 20 minute reserve that gives 12 minute flights.
And even THAT is over-stating it because we've assumed the whole flight is at economy cruise - no take-off or climb, and of course no taxiing. Is that useful?
PDR
* Most academic sources suggest that this is actually 60% rather than 80% unless you are happy to accept a reduced battery life