Food Calculation
Use the current level in the current ascension.
Food per hero = Base + (LV-2) * slope
1 Star 1000 + (LV-2) *20
2 Star 1200 + (LV-2) *24
3 Star 2100 + (LV-2) *42
4 Star 3000 + (LV-2) *60
5 Star 5000 + (LV-2) *100
So a 5 Star 4th ascension going from level 23 to 24 will take 5000 + 2100 food = 7100
This is true regardless of what level hero you feed to hero you are trying to level up. - so it is 7100 if you feed a 5 star or a 1 star to another hero.