As soon as caffeine is ingested, it is distributed fairly evenly throughout your body and because people differ widely in their body mass, most experts believe that the best measure of caffeine dosage is the one calculated per kilogram of body mass basis.
This is easily calculated by dividing the total amount of caffeine consumed by your body mass in kilos.
If you consume 200mg of caffeine and you have 80kg, your dose rate is 200/80 = 2.5mg/kg.
If you want to to achieve a specific dose rate then the total amount of caffeine you need to ingest is easily calculated by multiplying the dose rate you need by your weight in kg.
If you have 60kg and you want to achieve a dose rate of 3mg/kg, your total dose is 3 x 60 = 180mg.