Why the last tank is different
The last tank is usually only partly full. Put in as much product as in a full tank and the concentration is too high. The calculator reduces the last tank's dose in proportion to its water.
Dose per hectare or per 100 L of water
A dose per hectare (kg/ha, L/ha) does not depend on the water rate: the total is dose × area, and each tank gets a share in proportion to its water.
A concentration (g or ml per 100 L, %) follows the water: amount = water ÷ 100 × dose. A 0.2 % concentration means 0.2 L (or kg) of product per 100 L of water.
A helper, not a recommendation. The label always prevails: maximum dose, number of treatments, pre-harvest interval and restrictions.