Cow formula (cfr. italian expression 'formula bovina'):
epsilon=(4/3 PI R_vir^3)^(1/3)/N^(1/3)
where R_vir = virial radius of DM halo, N = particles in the simulation
Accurate formula (Dehnen 2001, MNRAS, 324, 273; see also Zhan, 2006, ApJ, 639, 617):
epsilon=0.017 r_s (N/10^5)^-0.23
where r_s is the scale radius (in a NFW model, r_s=r_vir/c) and N the total number of particles.