position sizing among different currencies

i trade all teh six currencies and i notice that there is a difference between the value of the gains/losses and margins offered by IB that i am not using the right position sizing. i am trading with the different margin levels for each instrument depending on thier notifications. the variables that i see are
base currency vs forex pair
forex pair x IB margin levels.

Assuming that i want to trade 1.5 k of base currency in each of the pair, how do i determine the correct lot sizing? Lets ignreo risk reward as i have it managed per trade.