Calculate the Flowing Wellhead Pressure by working backwards from the pressure at the sandface to give you the drawdown (and the production rate) you want up. Then calculate the FWP by working backwards from the inlet pressure required at the separator to the wellhead.
Hopefully (!) the caculated FWP from downhole will be higher than the pressure from the separator....the difference between the two is the pressure you want to drop across the choke.
Take this pressure (and the flowrate), plug it into the bean sizing software mentioned above, to get an idea of the bean size required.