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.
Another interesting fact - the food calc is the cost for one press of the level up button. If you give 10 heros to the food god (max) all 10 of them will cost the same amount - - it is true you might level up your hero by 13 levels - but each feeder hero costs the same! – if you fed them one at a time - the food cost would go up and be set by the current hero level when you press the initial level button.
So as a rule it saves food to always train with multiple heros.