The dosing estimate
This model treats EC as rising in a straight line with the amount of stock added and assumes each liter of stock in 100 liters of water raises EC by 1 mS/cm. Then dose = (target minus measured) x tank volume / 100.
Real stock strength depends on the recipe and EC is not perfectly linear at high concentration, so treat this as a simplified mass balance. That is also why controllers dose a little, mix, measure, and repeat.
Try it: set a 1,000 liter tank at 0.9 with a target of 1.6. The pumps need about 7 liters of stock. Halve the tank to 500 liters and the dose halves too. Then push the measured EC above the target and see the controller switch to dilution.
pH rides along
Most hydroponic crops do best with a solution pH around 5.5 to 6.5. Fertilizer and plant uptake both shift pH, so a third pump adds small amounts of acid, or sometimes base, to hold it in range.