Cho Ult and flash

If clicks for ult before you flashed there is nothing that can really be done to fix that. Basically the only thing that should be fix is that your flash went off but thats hard to fix with it being within milliseconds between his ult going off and you flashing.

You can't flash a unit-targeted ability; you can only flash skillshots. Cho'gath's Feast is enemy-targeted, so you can't flash it.

For example, you can't flash a Nunu ice ball, or Singed's flip, or Garen's ult, since these are enemy-targeted abilities. A full list of targeted abilities: http://leagueoflegends.wikia.com/wiki/Unit-targeted

But, you can flash a Cho'gath rupture, Nidalee spear, Lux laser, or Ashe arrow, since these are skillshots. List of direction skillshots: http://leagueoflegends.wikia.com/wiki/Direction-targeted List of ground skillshots: http://leagueoflegends.wikia.com/wiki/Ground-targeted

It has nothing to do specifically with Cho'Gath. It is the fact that his Ultimate is a "Targeted" ability rather than a Skillshot. Once the ability has picked its target and "goes off" it will hit regardless of if the target then moves out of range.

There is nothing to be fixed. It's a targetted ability. It's just one of those times where you wasted your flash, everyone has them just deal with it.