Question: how to calculate how much iron you need?
Answer: Iron deficit - equation derivation1 :
Assumptions: -Blood volume = 65 mL/kg. -Hgb conc target =14.0 g/dL. ... 
Intermediate calculations: -Blood volume (dL) = [65 (mL/kg) x body weight (kg)] / 100 (mL/dL) ... 
Final calculations: -Hgb iron deficit (mg) = weight (kg) x (14 - Hgb) x (2.145)