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.