Preprocessing with dwipreproc for 64 direction DWI image

Dear all,

I have read Andy’s book and it tells that “the options --slm=linear (which can be useful for data that was acquired with less than 60 directions)”. My data is 64 direction epi sequence obtained from 3T prisma Siemens. Would you kindly tell me which slm mention should I apply?
Thank all!