Hi there,
What kind of dose are you hoping to calculate: the air-kerma, the absorbed dose, the equivalent dose, the effective dose, the ambient dose H*(10), the ambient dose H*(0.07)? Then, will your neutron source give a homogeneous field, which body part will be exposed to it, what is the energy range of the neutron source, are also question that you must answer before even trying to make any kind of calculation. Just saying that you want to find a formula on how to calculate the dose for a neutron source for a human body is not like backing cookies: there is not one recipe for it.
You can start by looking at the unscear website. The IAEA and the ICRP website are also very good place to find information on the latest on radiation exposure.
Cheers