On a self-supported ride the question is never "how much per day" but "how much do I carry across the next gap without a shop or tap". Enter your longest resupply gap and get a rough carry target.
Rough planning estimates, not medical advice. Adjust to your own sweat rate, effort and appetite, and always keep a reserve for delays.
Rekker maps every shop, fuel stop and water point along your GPX and flags the longest carries for you. The Pro Carry planner does this per leg.
Time in the gap is distance divided by your average speed. Water is that time multiplied by an hourly rate that rises with heat (roughly 0.5 to 1.3 litres per hour). Food assumes about 300 kcal per hour of endurance riding, and we show it as ~250 kcal bars or portions.
Top up at every reliable tap even if your bottles are not empty. Cemeteries, graveyards and cemeteries with a tap are common water sources when the route runs dry. Treat "24/7" fuel stations as your night-time lifeline, and never assume a supermarket is open on a Sunday in Germany or the Alps.