He considers 1500 calories necessary for each day. Then has 400 days worth of food so far. 400*1500 = 600000 calories currently. If we spread that out 1425 days it would be around 421 cal for each day. So he needs to generate around 1080 additional calories(which he rounds to 1100 cal) per day if he wants to 1500 total for 1425 days?

Am i right or is it something more complicated?